
Book . ^3. 



23? 



M,'<>n 




CONVERSATIONS 

ON 
IN WHICH 

THE ELEMENTS OF THAT SCIENCE 

ARE FAMILIARLY EXPLAINED, 

AND 

ADAPTED TO THE COMPREHENSION OF 
YOUNG PUPILS. 

ILLUSTRATED WITH PLATES. 

BY THE AUTHOR OF CONVERSATIONS ON CHEMISTRY AND 
CONVERSATIONS ON POLITICAL ECONOMY 

APPROPRIATE QUESTIONS, 

FOR THE EXAMINATION OF SCHOLARS ; 

i 
ALSO BY 

AND 

A DICTIONARY OF PHILOSOPHICAL TERfllS. 



BY REV. J. L. BLAKE, A. M. 

Kector of St. Matthew* Church, and Principal of a Literary Seminary, 
Boston, Mass. 



BOSTON STEREOTYPE EDITION. 



Mouton : 

PRINTED AND PUBLISHED BY LINCOLN & EDMANDS, 

No. 59. Washington Street, (Cornhill,) 



1831. 



:b 



DISTRICT OF MASSACHUSETTS, to wit }& \ 

ulD District CM«rV« 0./M-. 

RF IT REMEMBERED, that on the fourth day of December, A. D. 1824, in 
the for tv-n nth year of the independence of the United States of America JOHN 
LA JR1S BLAIvE, of the said district, has deposited in this office the title of a 
book the ri-ht whereof he claims as author, in the word* following, to wtfr 

"Conversations on Natural Philosophy, in which the elements of that science 
are famU arly explained, and adapted to the comprehension of young pupils. 
Illustrated with plates. By the author of Conversations on Chemistry, and Con- 
versations^^ Economy. Improved by appropriate Rations for the 

examination of Scholars ; also by Illustrative Notes, and a Dictionar y of M"lojo- 
phical Terms. By J. L. BLAKE, A. M. Rector of St. Matthew's Church, and 
Principal ofa Literary Seminary, Boston, Mass." 

In Conformity to the Act of the Congress of the United States, entitled, « An 
Act for he encouragement of learning, by securing the copies of maps charts, 
am' books to the authors and proprietors of such copies during the times therein 
mentioned ■" and also to an Act, entitled -An Act supplementary to an Act, 
Sled An Act for the encouragement of learning, by securing the copies of 
nmns chfrts and books, to the authors and proprietors of such copies during the 
Ss therein mentioned: and extending the benefits thereof to the arts of de- 
signing, engraving, and etching historical, and other prin tn, ^ ^^ 
JNO. W. DAVIS, j f Massachusetts. 



NOTICE. 

This fascinating and familiar treatise on Natural Philosophy 
has probably contributed more to excite, in the minds of the 
young, a fondness for studying the science, than all other works 
together. The easy, natural, and striking illustrations, with 
which it abounds, awaken a deep interest, and rivet the atten- 
tion of the pupil. 

This edition has been introduced into the Female Depart- 
ment of the Publick Schools in Boston, with the happiest 
effect. Instructed have assured the publishers, that no study 
is pursued with equal gratification to the scholars. 

From numerous testimonials, the following recommendation 
is selected : — 

From the Rev. Jasper Mains, A.M., Principal and Professor of 

Mathematics and Natural Philosophy, in Charleston College, 

South Carolina. 

" I have been highly gratified with the perusal of your edition 
of Conversations on Natural Philosophy. The Questions, Notes 
and Explanations of Terms are valuable additions to the work, 
and make this edition superior to any other with which I am 
acquainted I shall recommend it wherever I have opportu- 
nity." 

Charlton, Jan. 10, 1826. 



PREFACE. 



The following work does not probably contain so much of the 
science of Natural Philosophy as might be crowded into a volume ot 
equal size, on some different plan. The author seems to have been 
influenced chiefly by other considerations ; and, in the opinion of 
the editor, with the most harppy success. Mrs. Marcet did not 
profess to prepare a work suited to the highest stages of education 
Her aim was to accommodate an important science to the literary 
taste and intellectual apprehensions of persons, within whose reach 
Natural Philosophy had not previously been placed — to accommo- 
date to the use of schools generally a science, which had hitherto 
been considered too abstruse and uninteresting for any, whose 
minds had not been disciplined and invigorated by long and regu 
lar habits of study. Instead of exhausting the intellectual ener 
gies of youth in committing to memory definitions and mathemati- 
cal demonstrations, which would not be understood, she proposed 
to illustrate the great principles of Natural Philosophy by compari- 
sons of the most familiar kind ; and, it is believed, Mrs. Marcet has 
done more, in this way, towards giving youth a taste for the study 
of philosophy than all others who have published treatises on the sub- 
ject. In her preface she remarks: — " It is with increased diffidence 
that the author offers this little work to the publick. The encou- 
raging reception which the Conversations on Chemistry and Politi- 
cal Economy have met with has induced her to venture on publish- 
ing a short course on Natural Philosophy. They are intended, in a 
course of elementary science, to precede the Conversations on 
Chemistry, and were actually written previous to either of her other 
publications." 

The Conversations on Natural Philosophy were introduced into 
the editor's Seminary about three years since, then at Concord, 
N. H. ; but it was soon found that his pupils were often embar- 
rassed in not knowing to what particular parts they were chiefly 
to direct the attention, committing to memory what was not neces- 
sary and omitting what was, thereby causing great loss of time as 
well as of improvement. This induced him to prepare, as they 
were needed, day after day, Questions for their examination, 
When questions were thus prepared upon the whole work, it was 



PREFACE. 

judged expedient to navo them published in a pamphlet, which 
was accordingly done ; but being prepared in haste and without 
thought of their being published, they were of course imperfect ; 
nor was there opportunity to revise them, when afterwards printed 
with notes in connexion with the work itself But as successive 
editions were required, and as the demand is still increasing, he has 
been induced to revise and write them anew, placing them at the 
bottom of the several pages to which they relate ; and, also to in- 
crease the number of Notes, and to add to the volume a Dictionary 
of Philosophical Terms. 

As the work is now presented to the publick, the Editor has 
full confidence in recommending it to Instructors, well persuaded 
it will lessen their own labour and facilitate the improvement of 
their pupils. It is perfectly obvious, that, instead of embodying, 
the questions at the close of the book, as in former impressions 
great convenience will be found, both by instructers and scholars, in 
having them printed on the pages from which they are to be an- 
swered ; nor is the labour of finding the answers to be given so les- 
sened, as to enable scholars to select those answers without read- 
ing and studying the whole book. 

It has been thought best to place the Plates at the end of the 
volume. If interspersed throughout the work, as in former edi- 
tions, it is evident that no more than one page could face each 
Plate, while a very considerable number of pages would have re- 
ference to it, so that the object contemplated could only in a small 
degree be accomplished. Besides, it is judged advisable by the 
editor, that the plates should not face the explanations in the 
Text if practicable. Many of the Questions are to be answered 
from the Plate* ; but if the several Plates were placed opposite the 
different portions of the work to which they relate, the answers 
might be read from the explanations there given instead of being 
recited from the figures as intended. 

J. L. BLAKE. 
Boston, December, 1824. 



CONTENTS. 



CONVERSATION I. 

On General Properties of Bodies. 

Introduction ; General Properties of Bodies ; Impenetrability; 
Extension ; Figure ; Divisibility ; Inertia ; Attraction ; Attrac- 
tion of Cohesion ; Density ; Rarity ; Heat ; Attraction of Gra- 
vitation. Page 9. 

CONVERSATION IL 

On the Attraction of Gravity. 

Attraction of Gravitation continued ; Of Weight ; Of the fall of* 
Bodies; Of the resistance of the Air ; Of the Ascent of Light 
Bodies. Page 24. 

CONVERSATION III. 

On the Latvs of Motion. 

Of Motion ; Of the Inertia of Bodies; Of Force to produce Mo- 
tion ; Direction of Motion ; Velocity, absolute and relative ; 
Uniform Motion ; Retarded Motion ; Accelerated Motion ; Ve- 
locity of Falling Bodies ; Momentum ; Action and Re-action 
equal ; Elasticity of Bodies ; Porosity of Bodies ; Reflected Mo 
tion ; Angles of Incidence and Reflection. Page 36. 

CONVERSATION IV. 

On Compound Motion. 

Compound Motion the result of two opposite forces; Of Circular 
Motion, the result of two forces, one of which confines the body 
to a fixed point ; Centre of Motion, the point at rest while 
the other parts of the body move round it ; Centre of Magnitude 
the middle of a body ; Centripetal Force, that which confines 
a body to a fixed central point ; Centrifugal Force,that which im- 
pels a body to fly from the centre ; Fall of Bodies in a Parabola ; 
Centre of Gravity, the Centre of Weight, or point about which 
the parts balance each other. Page 51 

1 * 



VI CONTENTS. 

CONVERSATION V 

On the Mechanical Powers. 

Of the Power of Machines ; Of the Lever in general ; Of the Le- 
ver of the first kind, having the Fulcrum between the Power and 
the weight ; Of the Lever of the second kind, having the 
weight between the power and the Fulcrum ; Of the Lever 
of the third kind, having the power between the Fulcrum and 
the Weight ; Of the Pulley ; Of the Wheel and Axle ; Of the 
inclined Plane; Of the Wedge ; Of the Screw. Pages 60, 68. 

CONVERSATION VI. 

ASTRONOMY. 

Causes of the Earth's Annual Motion. 

Of the Planets, and their motion ; Of the Diurnal motion of the 
Earth and Planets. Page 78. 

CONVERSATION VII. 

On the Planets, 

Of the Satellites or Moons ; Gravity diminishes as the square of 
the Distance; Of the Solar System; Of Comets; Constellations, 
signs of the Zodiack ; Of Copernicus, Newton, &c. Page 89. 

CONVERSATION VIII. 

On the Earth. 

Of the Terrestrial Globe ; Of the Figure of the Earth ; Of tho 
pendulum ; Of the Variation of the Seasons, and of the Length 
of Days and Nights ; Of the causes of the Heat of Summer ; 
Of Solar, Siderial, and Equal or Mean Time. Pago 102 

CONVERSATION IX. 

On the Moon. 

Of the Moon's Motion ; Phases of the Moon ; Eclipses of the 
Moon ; Eclipses of Jupiter's Moons ; Of the Latitude and Longi- 
tude , Of the transits of the inferior Planets ; Of the Tides. 

Page 124 



CONTENTS. v!i 

CONVERSATION X. 

HYDROSTATICKS. 

On the Mechanical Properties of Fluids. 

Definition of a Fluid ; Distinction between Fluids and Liquids ; 
Of Non-Elastic Fluids, scarcely susceptible of Compression ; 
Of the Cohesion of Fluids ; Of their Gravitation ; Of their Equi- 
librium : Of their Pressure ; Of Specifick Gravity ; Of the 
Specifick Gravity of Bodies heavier than Water ; Of those of 
the same weight as Water ; Of those lighter than Water ; Of 
the Specifick Gravity of Fluids. Page 137. 

CONVERSATION XL 

Of Springs, Fountains, &fc. 

Of the Ascent of Vapour and the Formation of Clouds ; Of the 
Formation and Fall of Rain, &c. ; Of the Formation of Springs ; 
Of Rivers and Lakes; Of Fountains. Page 150. 

CONVERSATION XII. 

PNEUMATICKS. 

On the Mechanical Properties of Air. 

Of the Spring or Elasticity of the Air ; Of the Weight of the Air ; 
Experiments with the Air Pump ; Of the Barometer ; Mode of 
Weighing Air ; Specific Gravity of Air ; Of Pumps ; Descrip- 
tion of the Sucking Pump ; Description of the Forcing Pump, 

Page 158. 

CONVERSATION XIII. 

On Wind and Sound. 

Of Wind in General; Of the Trade Wind ; Of the Periodical 
Trade Winds ; Of the Aerial Tides ; Of Sound in General ; 
Of Sonorous Bodies ; Of Musical Sounds ; Of Concord or Har- 
mony, and Melody. Page 170. 

CONVERSATION XIV. 

On Optics. 

Of Luminous, Transparent, and Opaque Bodies ; Of the Radiation 
of Light ; Of Shadows ; Of the Reflection of Light ; Opaque 
Bodies seen only by Reflected Light ; Vision Explained ; Ca- 
mera Obscura j Imago of Objects on the Retina. Page 183 



v in CONTENTS. 

CONVERSATION XV. 

On the Angle of Vision, and Reflection of Mirror j. 

Angle of Vision ; Reflection of Plain Mirrors ; Reflection of Con- 
vex Mirrors ; Reflection of Concave Mirrors. Page 197. 

CONVERSATION XVI. 

On Refraction and Colours. 

Transmission of Light by Transparent Bodies ; Refraction ; Re- 
fraction of the Atmosphere ; Refraction of a Lens ; Refraction 
of the Prism ; Of the Colours of Rays of Light ; Of the Colours 
of Bodies. Page 211. 

CONVERSATION XVII. 

OPTICKS. 

On the Structure of the Eye, and Optical Instruments. 

Description of the Eye ; Of the Image on the Retina ; Refraction 
of the Humours of the Eye; Of the Use »f Spectacles ; Of the 
Single Microscope : Of the Double Microscope ; Of the Solar 
Microscope ; Magick Lantern ; Refracting Telescope ; Reflect- 
ing Telescope. Page 229. 

A Dictionary of Philosophical Terms. Page 241. 



Direction to the Binder. 

The Plates, with the exception of the Frontispiece, 
which is to face the Title Page, to be put at the close 
of the volume, in their order of being numbered. 



CONVERSATION I. 



ON GENERAL PROPERTIES OF BODIES. 

Introduction ; General Properties of Bodies ; Impene- 
trability ; Extension ; Figure ; Divisibility ; Inertia ; 
Attraction ; Attraction of Cohesion ; Density; Rarity ; 
Heat ; Attraction of Gravitation. 

EMILY. 

I must request your assistance, my dear Mrs. B. in a 
charge which I have lately undertaken ; it is that of in- 
structing my youngest sister, a task, which I find proves 
more difficult than I had at first imagined. I can teach 
her the common routine of children's lessons tolerably 
well ; but she is such an inquisitive little creature, that 
she is not satisfied without an explanation of every diffi- 
culty that occurs to her, and frequently asks me questions 
which I am at a loss to answer. This morning, for in- 
stance, when I had explained to her that the world was 
round like a ball, instead of being flat as she had suppos- 
ed, and that it was surrounded by the air, she asked me 
what supported it. I told her that it required no sup- 
port; she then inquired why it did not fall as every 
thing else did. This I confess perplexed me ; for I had 
myself been satisfied with learning that the world floated 
in the air, without considering how unnatural it was that 
so heavy a body, bearing the weight of all other things, 
should be able to support itself. 

Mrs. B. I make no doubt, my dear, but that I shall 
be able to explain this difficulty to you ; but I believe 
that it would be almost impossible to render it intelligible 
to the comprehension of so young a child as your sister 
Sophia. You, who are now in your thirteenth year, may, 
I think, with great propriety, learn not only the cause of 
this particular fact, but acquire a general knowledge of 
Ihe laws by which the natural world is governed. 

Emily. Of all things it is what I should most like to 
learn ; but I was afraid it was too difficult a study even at 
my age. 



10 GENERAL PROPERTIES OF BODIES. 

Mrs. B. Not when familiarly explained ; if you nave 
patience to attend, I will most willingly give you all the 
information in my power. You may perhaps find the 
subject rather dry at first ; but if I succeed in explaining 
the laws of nature, so as to make you understand them, I 
am sure that you will derive not only instruction, but 
great amusement from that study. 

Emily. I make no doubt of it, Mrs. B. ; and pray 
begin by explaining why the earth requires no support ; 
for that is the point which just now most strongly excites 
my curiosity. 

Mrs. B. My dear Emily, if I am to attempt to give 
you a general idea of the laws of nature, which is no less 
than to introduce you to a knowledge of the science of 
natural philosophy, it will be necessary for us to proceed 
with some degree of regularity. I do not wish to confine 
you to the systematic order of a scientific treatise ; but if 
we were merely to examine every vague question that 
may chance to occur, our progress would be but very slow. 
Let us, therefore, begin by taking a short survey of the 
general properties of bodies, some of which must necessa- 
rily be explained before I can attempt to make you under 
, stand why the earth requires no support. 
/ When I speak of bodies, I mean substances, of what 
ever nature, whether solid or fluioA^and matter is the ge- 
neral term used to denote the substancej^whatever itj» 
nature be, of which the different bodies are composed./ 
Thus, wood is the matter of which this table is made , 
water is the matter with which this glass is filled, &c. 

Emily. I am very glad you have explained the mean- 
ing of the word matter, as it has corrected an erroneous 
conception I had formed of it : I thought that it was ap- 
• plicable to solid bodies only. 

Mrs. B.y There are certain properties which appear 

to be common to all bodies, and are hence called the cs- 

sential properties of bodies ; these are, Impenetrability , 

Extension, Figure, Divisibility, Inertia, and Attraction^ 

These are called the general properties of bodies, as wfc*' 



do not suppose any body to exist without them. 



1 



1. What is to be understood by the term bodies, as used in phi- 
losophy ? 2. What term is used to denote substances ? 3. 

What properties are common to all bodies ? 4. Why are 

these called general properties of bodies ? 



V 



GENERAL PROPERTIES OF BODIES. 11 

, 1/ By impenetrability , is meant the property which bodies 
nave of occupying a certain space, so that, where one 
body is, another cannot be, without displacing the for- 
mer ; for two bodies cannot exist in the same place at the 
same time. ■;)£& liquid may be more easily removed than 
a solid body ; yet it is not the less substantial, since it is 
as impossible for a liquid and a solid to occupy the same 
space at the same time, as for two solid bodies to do so. 
For instance, if you put a spoon into a glass full of water, _fe? 
the water will flow over to make room for the spoon. 

Emily. I understand this perfectly. Liquids are in 
reality as substantial or as impenetrable as solid bodies, 
and they appear less so, only because they are more ea- 
sily displaced. 

Mrs. B. The air is a fluid differing in its nature from 
liquids, but no less impenetrable. If I endeavour to fill 

7 this phial by plunging it into this bason of water, the air, 
you see, rushes out of the phial in bubbles, in order to 
make way for the water, for the air and the water cannot 
exist together in the same space, any more than two 
hard bodies y and if I reverse this goblet, and plunge it per- 
pend iculafTy into the water, so that the air will not be able 
to escape, the water will no longer be able to fill the goblet. 
r Emily. But it rises a considerable way into the glass. 
j~^ Mrs. B. Because the water compresses or squeezes 
the air into a small space in the upper part of the glass ; 
but, as long as it remains there, no other body can occu- 
py the same place. 

Emily. A difficulty has just occurred to me, with re- 
gard to the impenetrability of solid bodies ; if a nail is 
driven into a piece of wood, it penetrates it, and both 
the wood and the nail occupy the same space that the 
wood alone did before. 

Mrs. B. The nail penetrates between the particles of 
the wood, by forcing them to make way for it ; for you 
know that not a single atom of wood can remain in the 
space which the nail occupies ; and if the wood is not 
increased in size by the addition of the nail, it is because 
wood is a porous substance, like sponge, the particles of 

5. What is impenetrability ? 6. Can liquids occupy the 

same space of a solid body ? 7. How can you prove that they 

cannot occupy the same space occupied by solids ? 8. Can Li- 
quids and air occupy the same space in the same time ? 9. How 

would you prove that they cannot ? 



12 GENERAL TROrERTIES OF BODIES. 

which may be compressed or squeezed closer together • 
and it is thus that they make way for the nail. 

We may now proceed to the next general property oi 
bodies, extension. A body which occupies a certain 
space must necessarily have extension ; that is to say, 
length, breadth, and depth ; these are called the dimen- 
sions of extension ; can you form an idea of any body 
without them 1 

Emily. No : certainly I cannot ; though these dimen- 
sions must, of course, vary extremely in different bodies. 
The length, breadth, and depth, of a box, or of a thimble, 
are very different from those of a walking-stick, or of a 
hair. 

But is not height also a dimension of extension ? 

Mrs. B. Height and depth are the same dimension, 
considered in different points of view ; if you measure a 
body, or a space, from the top to the bottom, you call it 
depth ; if from the bottom upwards, you call it height ; 
thus the depth and height of a box are, in fact, the same 
thing. 

Emily. Very true ; a moment's consideration would 
haye enabled me to discover that ; and breadth and 
width are also the same dimension. 

iMrs. B. Yes ; the limits of extension constitute fi- 
gure or shape. You conceive that a body having length, 
breadth, and depth, cannot be without form, either sym- 
metrical or irregular. 

\Emily. Undoubtedly ; and this property admits of al- 
most an infinite variety. 

Mrs. B. Nature has assigned regular forms to her 
productions in general. The natural form of mineral sub- 
stances is that of crystals, of which there is a great variety. 
Many of them are very beautiful, and no less remarkable 
by their transparency, or colour, than by the perfect 
regularity of their forms, as may be seen in the various 
museums and collections of natural history. The vege- 
table and animal creation appears less symmetrical, but is 
still more diversified in figure than the mineral kingdom. 

10. What is meant by extension ? 11. What is the differ- 
ence between height and depth as applied to extension? 12 

What is the firrure of a body ? 13. What forms has nature, in 

iven to her productions ? 14. What is said of mine 

15. How doofl the vegetable and animal ere 

n compare with the mineral klngdo] 



GENERAL PROPERTIES OF BODIES. 13 

Manufactured substances assume the various arbitrary 
forms which the art of man designs for them ; and an in- 
finite number of irregular forms are produced by frac- 
tures, and by the dismemberment of the parts of bodies. 

Emily. Such as a piece of broken china or glass ? 

Mrs. B. Or the fragments of mineral bodies which are 
broken in being dug out of the earth, or decayed by the 
effect of torrents and other causes. The picturesque ef- 
fect of rock-scenery is in a great measure owing to acci- 
dental irregularities of this kind. 

We may now proceed to divisibility ; that is to say, a 
susceptibility of being divided into an indefinite number of 
parts. Take any small quantity of matter, a grain of sand 
for instance, and cut it into two parts ; these two parts 
might be again divided, had we instruments sufficiently 
fine for the purpose ; and if, by means of pounding, grind- 
ing and other similar methods, we carry this division to 
the greatest possible extent, and reduce the body to its 
finest imaginable particles, yet not one of the particles will 
be destroyed, and the body will continue to exist, though 
in this altered state. 

The melting of a solid body in a liquid affords a very 
striking example of the extreirie divisibility of matter ; 
when you sweeten a cup of tea, for instance, with what 
minuteness the sugar must be divided to be diffused 
throughout the whole of tlue liquid. 

Emily. And if you pour a few drops of red wine into 
a glass of water, they immediately tinge the whole of the 
water, and must therefore be diffused throughout it. 

Mrs. B. Exactly so ; and the perfume of this laven- 
der water will be almost as instantaneously diffused 
throughout the room, if I take out the stopper. 

Emily. But in this case it is only the perfume of the la- 
vender, and not the water itself, that is diffused in the room ? 

Mrs. B. The odour or smell of a body is part of the 
body itself, and is produced by very minute particles or 
exhalations which escape from odoriferous bodies. It 
would be impossible that you should smell the lavender 
water, if particles of it did not come in actual contact 
with your nose. 

16. What is divisibility in natural philosophy ? 17. What 

arc instances of practical divisibility of matter to a great ex- 
tent ? — -—18. On what principle is it that we can smell odorife- 
rous objects ? 

2 



1 1 GENERAL PROPERTIES OP BODIES* 

Emily. But when I smell a flower, I sec no vapour 
rise from it ; and yet I can perceive the smell at a con- 
siderable distance. 

Mrs. B. You could, I assure you, no more smell a 
flower, the odoriferous particles of which did not touch 
your nose, than you could taste a fruit, the flavoured par- 
ticles of which did not come in contact with your tongue. 

Emily. That is wonderful indeed ; the particles, then, 
which exhale from the flower and from the lavender water 
are, I suppose, too small to be visible ? 

Mrs. B. Certainly : you may form some idea of 
their extreme minuteness from the immense number 
which must have escaped in order to perfume the whole 
room ; and yet there is no sensible diminution of the 
liquid in the phial. 

Emily. But the quantity must really be diminished 1 

Mrs. B. Undoubtedly ; and were you to leave the 
bottle open a sufficient length of time, the whole of the 
water would evaporate and disappear. But though so 
minutely subdivided as to be imperceptible to any of our 
senses, each particle would continue to exist ; for it is 
not within the power of man to destroy a single particle 
of matter : nor is there any reason to suppose that in na- 
ture an atom is ever annihilated. 

Emily. Yet, when a body is burnt to ashes, part of it, 
at least, appears to be effectually destroyed 1 Look how 
small is the residue of ashes beneath the grate, from all 
the coals which have been consumed within it. 

Mrs. B. That part of the coals, which you suppose 
to be destroyed, evaporates in the form of smoke and va- 
pour, whilst the remainder is reduced to ashes. A body, 
in burning, undergoes no doubt very remarkable changes ; 
it is generally subdivided ; its form and colour altered ; 
its extension increased ; but the various parts, into which 
it has been separated by combustion, continue in exist- 
ence, and retain all the essential properties of bodies. 

Emily. But that part of a burnt body which evapo- 
rates in smoke has no figure ; smoke, it is true, ascends 

19. If we inhale particles of odoriferous objects, why cannot 

wo see these particles ? 20. If the particles of fragrant liquid 

in a phial escape from the phial in order to perfume a room, why 

can we not see them escape ? 21. Is not the matter, of which 

wood is composed, destroyed or. annihilated, when burnt to 
ashes ? 



GENERAL PROPERTIES OF BODIES. 15 

in columns into the air, but it is soon so much diffused as 
to lose all form ; it becomes indeed invisible. 

Mrs. B. Invisible, I allow ; but we must not imagine 
that what we no longer see no longer exists. Were every 
particle of matter that becomes invisible annihilated, the 
world itself would in the course of time be destroyed. 
The particles of smoke, when diffused in the air, continue 
still to be particles of matter, as well as when more closely 
united in the form of coals : they are really as substantial 
in the one state as in the other, and equally so when by 
their extreme subdivision they become invisible. No 
particle of matter is ever destroyed : this is a principle 
you must constantly remember. Every thing in nature 
decays and corrupts in the lapse of time. We die, and 
our bodies moulder to dust ; but not a single atom of 
them is lost ; they serve to nourish the earth, whence, 
while living, they drew their support.* 

The next essential property of matter is called inertia ; 
this word expresses the resistance which inactive matter 
makes to a change of state. Bodies appear to be equally 
incapable of changing their actual state, whether it be 
of motion or of rest. You know that it requires force 
to put a body which is at rest in motion ; an exertion 
of strength is also requisite to stop a body which is 
already in motion. The resistance of the body to a 
change of state, in either case, is called its inertia. 

Emily. In playing at base-ball I am obliged to use 
all my strength to give a rapid motion to the ball ; and 
when I have to catch it, I am sure I feel the resistance 



* As a further illustration of the great practical divisi- 
bility of matter, it may be added, that a single grain of gold may 
be hammered by a gold-beater until it will cover fifty square 
inches. Each square inch may then be divided into two hundred 
strips, and each strip into two hundred parts, which may be seen 
with the naked eye ; consequently, a square inch contains forty 
thousand visible parts, which multiplied by 50, the number of 
square inches which a grain of gold will make, give two million 
parts, which may be seen with the naked eye. — It has also been 
calculated, that sixteen ounces of gold, which, in the form of a 
cube, would not measure one inch and a quarter in its side, will 
completely gild a quantity of silver wire sufficient to surround 
the globe. 

22. Is it a principle in natural philosophy that no particle of 

matter can be destroyed ? 23. What is meant by the term 

inertia ? 24. What instances of great practical divisibility of 

matter are given in the note f 



16 GENERAL PROPERTIES OF BODIES. 

it makes to being stopped. But if I did not catch it, it 
would soon fall to the ground and stop of itself. 

3Irs. B. Inert matter is as incapable of stopping of it- 
self, as it is of putting itself in motion: when the ball ceases 
to move, therefore, it must be stopped by some other cause 
or power ; but as it is one with which you are yet un- 
acquainted, we cannot at present investigate its effects. 

The last property which appears to be common to all 
bodies is attraction. All bodies consist of infinitely small 
particles of matter, each of which possesses the power of 
attracting or drawing towards it, and uniting with any 
other particle sufficiently near to be within the influence 
of its attraction ; but in minute particles this power ex- 
tends to so very small a distance around them that its 
effect is not sensible, unless they are (or at least appear 
to be) in contact; itjthen makes them stick or adhere 
together, and is hence called the attraction of cohesion. 
Without this power, solid bodies would fall in pieces, or 
rather crumble to atoms. 

Emily. I am so much accustomed to see bodies firm and 
solid, that it never occurred to me that any power was 
requisite to unite the particles of which they are composed. 
But the attraction of cohesion does not, I suppose, exist 
in liquids ; for the particles of liquids do not remain to- 
gether so as to form a body, unless confined in a vessel ? 

Mrs. B. I beg your pardon ; it is the attraction of 
cohesion which holds this drop of water suspended at the 
end of my finger, and keeps the minute watery particles 
of which it is composed united. But as this power is 
stronger in proportion as the particles of bodies are more 
closely united, the cohesive attraction of solid bodies 
is much greater than that of fluids. The thinner and 
lighter a fluid is, the less is the cohesive attraction of 
its particles, because they are further apart ; and in elastic 
fluids, such as air, there is no cohesive attraction among 
the particles. 

25. What would be the consequence, if a body were put in 

motion and no resistance should be offered ? 2G. What is the 

propertv common to all bodies ? 27. Of what do all bodies 

consist? 28. What is the power called which binds these 

small particles together ? 29. What would be the conse- 
quence if the power of cohesive attraction were destroyed ? 

30. Does the power of cohesion exist also in liquids ? 31. 

How would you prove that it exists in liquids ? 39. Why are 

gome bodies hard and others soft ? 



GENERAL PROPERTIES OF BODIES. 17 

Emily. That is very fortunate ; for it would be im- 
possible to breathe the air in a solid mass ; or even in 
a liquid state. But is the air a body of the same nature 
as other bodies ? 

Mrs. B. Undoubtedly, in all essential properties. 

Emily. Yet you say that it does not possess one of 
the general properties of bodies — cohesive attraction ? 

Mrs. B. The particles of air are not destitute of the 
power of attraction, but they are too far distant from each 
other to be influenced by it ; and the utmost efforts of 
human art have proved ineffectual in the attempt to com- 
press them, so as to bring them within the sphere of each 
other's attraction, and make them cohere. 

Emily. If so, how is it possible to prove that they are 
endowed with this power ? 

Mrs. B. The air is formed of particles precisely of 
the same nature as those which enter into the composi- 
tion of liquid and solid bodies, in which state we have a 
proof of their attraction. 

Emily. It is then, I suppose, owing to the different 
degrees of attraction of different substances, that they are 
hard or soft ; and that liquids are thick or thin 1 

Mrs. B. Yes ; but you would express your meaning 
better by the term density, which denotes the degree of 
closeness and compactness of the particles of a body : 
thus you may say, both of solids, and of liquids, that the 
stronger the cohesive attraction the greater is the den- 
sity of the body. In philosophical language, density 
is said to be that property of bodies by which they con- 
tain a certain quantity of matter, under a certain bulk or 
magnitude. Rarity is the contrary of density ; it denotes 
the thinness and subtlety of bodies : thus you would say 
that mercury or quicksilver was a very dense fluid ; 
ether, a very rare one, &c. 

Caroline. But how are we to judge of the quantity of 
matter contained in a certain bulk ? 



33. Does the attraction of cohesion exist in the air ? — 34. But 
are the particles of the air actually under the influence of this 

attraction ? 35. Why are they not, if attraction belong to 

them ? 36. How do we know that attraction does belong to 

the air if no influence is exerted upon it ? 37. What is meant 

by the term density ? 38. What is meant by the term rarity ? 

2 * 



18 GENERAL PROPERTIES OF BODIES. 

Mrs. B. By the weight : under the same bulk bodies 
are said to be dense in proportion as they are heavy. 

Emily. Then we may say that metals are dense bodies, 
wood comparatively a rare one, 6lc. But, Mrs, B., when 
the particles of a body are so near as to attract each other, 
the effect of this power must increase as they are brought 
by it closer together ; so that one would suppose that the 
body would gradually augment in density, till it was im- 
possible for its particles to be more closely united. Now we 
know that this is not the case ; for soft bodies, such as cork, 
sponge, or butter, never become, in consequence of the in- 
creasing attraction of their particles, as hard as iron ? 

Mrs. B. In such bodies as cork and sponge, the parti- 
cles which come in contact are so few as to produce but a 
slight degree of cohesion ; they are porous bodies, which, 
owing to the peculiar arrangement of their particles, abound 
with interstices which separate the particles ; and these 
vacancies are filled with air, the spring or elasticity of 
which prevents the closer union of the parts. But there is 
another fluid much more subtle than air, which pervades all 
bodies, this is heat. Heat insinuates itself more or less be- 
tween the particles of all bodies, and forces them asunder ; 
you may therefore consider heat and the attraction of co- 
hesion, as constantly acting in opposition to each other. 

Emily. The one endeavouring to rend a body to 
pieces, the other to keep its parts firmly united. 

Mrs. B. And it is this struggle between the contend- 
ing forces of heat and attraction, which prevents the ex- 
treme degree of density which would result from the sole 
influence of the attraction of cohesion. 

Emily. The more a body is heated then, the more its 
particles will be separated. 

Mrs. B. Certainly ; we find that bodies swell or dilate 
by heat : this effect is very sensible in butter, for instance, 
which expands by the application of heat : till at length 

39. How are we to judge of the quantity of matter in bodies ? 
-40. In what proportion are bodies dense of the same bulk ? 



41. What bodies are usually said to be dense? 42. What 

ones are said to be rare ? 43. Why are not sponge and cork 

and other similar substances hard, since their particles come in 

contact ? 44. What fluid is named more subtle than air ? 

45. What effect has heat on bodies ? 40. What two forces 

are said to act always on bodies in opposition to each other ? 

47. In what cases may we sec the effect of heat in the expan- 
sion of bodies, or in the separation of their particles ? 



GENERAL PROPERTIES OF BODIES. 19 

the attraction of cohesion is so far diminished that the par- 
ticles separate, and the butter becomes liquid. A similar 
effect is produced by heat on metals, and all bodies sus- 
ceptible of being melted. Liquids, you know, are made 
to boil by the application of heat : the attraction of cohe- 
sion then yields entirely to the expansive power ; the 
particles are totally separated and converted into steam 
or vapour. But the agency of heat is in no body more sen- 
sible than in air, which dilates and contracts by its in- 
crease or diminution in a very remarkable degree.* 

Emily. The effects of heat appear to be one of the 
most interesting parts of natural philosophy. 

Mrs. B. That is true ; but heat is so intimately con- 
nected with chemistry, that you must allow me to defer 
the investigation of its properties till you become ac- 
quainted with that science. 

To return to its antagonist, the attraction of cohesion ; 
it is this power which restores to vapour its liquid form, 
which unites it into drops when it falls to the earth in a 
shower of rain, which gathers the dew into brilliant gems 
on the blades of grass. 

Emily. And I have often observed that after a shower, 
the water collects into large drops on the leaves of 
plants ; but I cannot say that I perfectly understand how 
the attraction of cohesion produces this effect. 

Mrs. B. Rain does not fall from the clouds in the form 
of drops, but in that of mist or vapour, which is composed 
of very small watery particles ; these in their descent, 
mutually attract each other, and those that are sufficient- 
ly near in consequence unite and form a drop, and thus 

* The expansive power of heat produces some of the most in- 
teresting phenomena in nature. The boiling of liquids, is the im- 
mediate result of this power ; and the operation, although simple, 
is peculiarly worthy of notice. As the numerous particles become 
expanded or rarified, they are continually rising to, and escaping 
from the surface, which occasions an agitation of the liquid, pro- 
portioned ? in its violence, to the degree of heat operating on 
it. — And on exposing our hands or other limbs to the fire, the 
internal fluid becomes expanded, which causes them to appear 
swollen ; whereas, when exposed to the cold, the abstraction of 
the heat causes them to be compressed. 

48. How are liquids made to boil by heat ; or how is the mo- 
tion or agitation of boiling liquids produced f 49. Why are 

/)ur hands and fingers sioollen or larger on being held near the 

fire, than when exposed to the cold ? 50. In what state does 

rain fall from the clouds ? 51. What collects this mist or 

vapour into drops ? 



20 GENERAL PROPERTIES OP BODIES. 

the mist is transformed into a shower. The clew also was 
originally in a state of vapour, but is, by the mutual at- 
traction of the particles, formed into small globules on the 
blades of grass : in a similar manner the rain upon the 
leaf collects into large drops, which, when they become too 
heavy for the leaf to support, fall to the ground. 

Emily. All this is wonderfully curious ! I am almost 
bewildered with surprise and admiration at the number 
of new ideas I have already acquired. 

Mrs. B. Every step that you advance in the pursuit 
of natural science, will fill your mind with admiration and 
gratitude towards its Divine Author. In the study of 
natural philosophy, we must consider ourselves as read- 
ing the book of nature, in which the bountiful goodness 
and wisdom of God is revealed to all mankind^ no study 
can then tend more to purify the heart, and raise it to a 
religious contemplation of the Divine perfections. 

There is another curious effect of the attraction of co- 
hesion which I must point out to you. It enables liquids 
to rise above their level in capillary tubes ; these are 
tubes, the bores of which are so extremely small that li- 
quids ascend within them, from the cohesive attraction 
between the particles of the liquid and the interiour sur- 
face of the tube. Do you perceive the water rising above 
its level in this small glass tube, which I have immersed 
in a goblet full of water 1 

Emily. Oh yes ; I see it slowly creeping up the tube, 
but now it is stationary ; will it rise no higher ? 

Mrs. B. No ; because the cohesive attraction be- 
tween the water and the internal surface of the tube is 
now balanced by the weight of the water within it : if the 
bore of the tube were narrower, the water would rise 
higher ; and if you immerse several tubes of bores of dif- 
ferent sizes, you will see it rise to different heights in 
each of them. In making this experiment, you should 
colour the water with a little red wine, in order to render 
the effect more obvious. 

All porous substances, such as sponge, bread, linen, 
&c. may be considered as collections of capillary tubes : 
if you dip one end of a lump of sugar into water, the 

52. What causes the dew on leaves and blades of grass to 

collect into drops ? 53. Why will liquids rise above their level 

in capillary tubes ? 54. On what principle do sponge, and 

other porous substances absorb liquids ? 



GENERAL PROPERTIES OF BODIES. 21 

water will rise in it ; and wet it considerably above the 
surface of that into which you dip it. 

Emily, In making tea I have often observed that 
effect without being able to account for it. 

Mrs. B. Now that you are acquainted with the at- 
traction of cohesion, I must endeavour to explain to you 
that of Gravitation, which is a modification of the same 
power ; the first is perceptible only in very minute parti- 
cles, and at very small distances ; the other acts on the 
largest bodies, and extends to immense distances. 

Emily. You astonish me : surely you do not mean to 
say that large bodies attract each other. 

Mrs. B. Indeed I do : let us take, for example, one 
of the largest bodies in nature, and observe whether it 
does not attract other bodies. What is it that occasions 
the fall of this book, when I no longer support it ? 

Emily. Can it be the attraction of the earth ? I 
thought that all bodies had a natural tendency to fall. 

Mrs. B. They have a natural tendency to fall, it is 
true ; but that tendency is produced entirely by the at- 
traction of the earth ; the earth being so much larger 
than any body, on its surface, forces every body, which 
is not supported, to fall upon it* 

Emily. If the tendency which bodies have to fall 
results from the earth's attractive power, the earth itself 
can have no such tendency, since it cannot attract itself, 
and therefore it requires no support to prevent it from 
falling. Yet the idea that bodies do not fall of their own 
accord, but that they are drawn towards the earth by its 
attraction, is so new and strange to me, that I know not 
how to reconcile myself to it. 

Mrs. B. When you are accustomed to consider the 
fall of bodies as depending on this cause, it will appear 
to you as natural, and surely much more satisfactory, than 
if the cause of their tendency to fall were totally unknown. 
Thus you understand, that all matter is attractive, from 
the smallest particle to the largest mass ; and that bodies 
attract each other with a force proportional to the quan- 
tity of matter they contain. 

Emily. I do not perceive any difference between the 
attraction of cohesion and that of gravitation : is it not be- 

55. What is the difference between cohesive attraction and 
gravitation ? 5 6. What causes bodies to fall to the earth ? 
, 57. In what proportion do bodies gravitate towards or at- 
tract each other ? 



22 GENERAL PROPERTIES OF BODIES. 

cause every particle of matter is endowed with an attrac- 
tive power, that large bodies, consisting of a great num- 
ber of particles, are so strongly attractive 1 

Mrs. B. True. There is, however, this difference 
between the attraction of particles and that of masses, that 
the former is stronger than the latter, in proportion to the 
quantity of matter. Of this you have an instance in the 
attraction of capillary tubes, in which liquids ascend by 
the attraction of cohesion, in opposition to that of gravity. 
It is on this account that it is necessary that the bore of 
the tube should be extremely small ; for if the column of 
water within the tube is not very minute, the attraction 
would not be able either to raise or support its weight, in 
opposition to that of gravity. 

You may observe, also, that all solid bodies are enabled 
by the force of the cohesive attraction of their particles 
to resist that of gravity, which would otherwise disunite 
them, and bring them to a level with the ground, as it 
<loes, in the case of liquids, the cohesive attraction of which 

is not sufficient to enable it to resist the power of gravity.* 

— * 

* The power of gravitation is greatest at the surface of the 
earth, whence it decreases both upwards and downwards ; but 
not in the same proportion. The force of gravity upwards is as 
the square of the distance from the centre. That is, gravity at 
the surface of the earth, which is about 4000 miles from the cen- 
tre, is four times more powerful than it would be at double that 
distance, or 8000 miles from the centre. Gravity and weight may 
be taken, in particular circumstances, as synonymous terms. We 
say, a piece of lead weighs a pound, or sixteen ounces ; but if by 
any means it could be carried 4000 miles above the surface of the 
earth, it would weigh only one fourth of a pound, or four ounces ; 
and if it could be transported to 8000 miles above the earth, 
which is three times the distance from the centre that the surface 
is, it would weigh only one ninth of a pound, or something less 
than two ounces. 

And it is demonstrated, that the force of gravity downwards de- 
creases, as the distance from the surface increases, so that at one 
half the distance from the centre to the surface / the same weight 



58. What example is given to show that cohesive attraction is 
stronger than gravitation ? 59. Why must the bore of capil- 
lary tubes be exceedingly small for water to rise in them ? 

60. What would be the effect of gravitation on bodies, were it not 

for cohesive attraction ? 61. Where is the power of gravity 

greatest ? 62. In what proportion does gravity decrease from 

the surface of the earth upwards f 63. In what proportion 

docs it decrease downwards ? 



GENERAL PROPERTIES OF BODIES. 23 

Emily. And some solid bodies appear to be of this 
nature, as sand and powder for instance : there is no at- 
traction of cohesion between their particles 1 

Mrs. B. Every grain of powder or sand is composed 
of a great number of other more minute particles, firmly 
united by the attraction of cohesion ; but amongst the 
separate grains there is no sensible attraction, because 
they are not in sufficiently close contact. 

Emit. Yet they actually touch each other ? 

Mrs. B. The surface of bodies is in general so rough 
and uneven, that when in actual contact, they touch each 
other only by a few points. Thus, if I lay upon the table 
this book, the binding of which appears perfectly smooth, 
yet so few of the particles of its under surface come in 
contact with the table, that no sensible degree of cohesive 
attraction takes place ; for you see, that it does not stick, 
or cohere to the table, and I find no difficulty in lifting 
it off. 

It is only when surfaces perfectly flat and well polished 
are placed in contact, that the particles approach in suffi- 
cient number, and closely enough, to produce a sensible 
degree of cohesive attraction. Here are two hemispheres 
of polished metal, I press their flat surfaces together, hav 
ing previously interposed a few drops of oil, to fill up 
every little porous vacancy. Now try to separate them. 

already described would weigh only one half of a pound, and so 
on — Thus, a piece of metal weighing, on the surface of the earth, 
one pound, will 

At the centre weigh - - - 

1000 miles from the centre, 1-4 pound. 

2000 1-2 

3000 % - 3-4 

4000 1 

8000 -------- 1-4 

12,000 ------- 1-9 

And at the distance of the moon from the earth which is 
240,000 miles, it would weigh only the 3, 600th part of a pound, 
because the distance is 60 times further from the centre of the 
earth than the surface. 

64. If a body weigh one pound at the surface of the earth, 
what will be its weight at the centre— at 1000— a* 2000— at 3000 
—at 4000— at 8000— and at 12,000 miles from the centre of it f 

65. What is the reason that cohesive attraction does not ope- 
rate on different bodies brought into contact, as well as on the 

particles of the same body ? 66. When will the surfaces of 

different bodies adhere to each other by the force of cohesivs 
attraction r 



24 ON THE ATTRACTION 01 GRAVITY. 

Emily. It requires an effort beyond my strength, 
though there are handles for the purpose of pulling them 
asunder. Is the firm adhesion of the two hemispheres, 
merely owing to the attraction of cohesion ? 

Mrs. B. There is no force more powerful, since it is 
by this that the particles of the hardest bodies are held 
together. It would require a weight of several pounds, 
to separate these hemispheres. 

Emily. In making a kaleidoscope, I recollect that the 
two plates of glass, which were to serve as mirrors, stuck 
so fast together, that I imagined some of the gum I had 
been using had by chance been interposed between them ; 
but now I make no doubt but that it was their own natu- 
ral cohesive attraction which produced this effect. 

Mrs. B. Very probably it was so ; for plate-glass has 
an extremely smooth, flat surface, admitting of the con- 
tact of a great number of particles, between two plates, 
laid one over the other. 

Emily. But, Mrs. B. the cohesive attraction of some 
bodies is much greater than that of others ; thus, glue, 
gum, and paste, cohere with singular tenacity. 

Mrs. B. That is owing to the peculiar chemical pro- 
perties of those bodies, independently of their cohesive at- 
traction. 

There are some other kinds of modifications of attrac- 
tion peculiar to certain bodies ; namely, that of magnet- 
ism, and of electricity ; but we shall confine our attention 
merely to the attraction of cohesion and of gravity ; the 
examination of the latter we shall resume at our next 
meeting. 



CONVERSATION II. 

ON THE ATTRACTION OF GRAVITY. 

Attraction of Gravitation, continued ; Of Weight ; Of 
the Fall of Bodies ; Of the Resistance of the Air ; Of 
the Ascent of Light Bodies. 

EMILY. 

I have related to my sister Caroline all that you have 
taught me of natural philosophy, and she has been so 
much delighted by it, that she hopes you wilJ have the 
goodness to admit her to your lessons. 



ON THE ATTRACTION OF GRAVITY. 25 

Mrs. B. Very willingly ; but I did not think you had 
any taste for studies of this nature, Caroline ? 

Caroline. I confess, Mrs. B., that hitherto I had form- 
ed no very agreeable idea, either of philosophy, or philo- 
sophers ; but what Emily has told me, has excited my curi- 
osity so much, that I shall be highly pleased if you will 
allow me to become one of your pupils. 

Mrs. B. I fear that I shall not find you so tractable a 
scholar as Emily ; I know that you are much biassed in 
favour of your own opinions. 

Caroline. Then you will have the greater merit in re- 
forming them, Mrs. B. ; and after all the wonders that 
Emily has related to me, I think I stand but little chance 
against you and your attractions. 

Mrs. B. You will, I doubt not, advance a number of 
objections ; but these I shall willingly admit, as they will 
be a means of elucidating the subject. Emily, do you 
recollect the names of the general properties of bodies 1 

Emily. Impenetrability, extension, figure, divisibility, 
inertia, and attraction. 

Mrs. B. Very well. You must remember that these 
are properties common to all bodies, and of which they 
cannot be deprived ; all other properties of bodies are 
called accidental, because they depend on the relation or 
connexion of one body to another. 

Caroline. Yet surely, Mrs. B., there are other proper 
ties which are essential to bodies, besides those you have 
enumerated. Colour and weight, for instance, are com- 
mon to all bodies, and do not arise from their connexion 
with each other, but exist in the bodies themselves ; these, 
therefore, cannot be accidental qualities. 

Mrs. B. I beg your pardon ; these properties do not 
exist in bodies independently of their connexion with 
other bodies. 

Caroline. What ! have bodies no weight ? Does not 
this table weigh heavier than this book ; and, if one thing 
weighs heavier than another, must there not be such a 
thing as weight ? 

Mrs, B. No doubt : but this property does not appear 
to be essential to bodies ; it depends upon their connex- 

67. What were the names of the common or general properties 
of bodies given in the first Conversation ?— - — 68. What are called 

the accidental properties of bodies ? <6d. Are colour and weight 

common or accidental properties ? 

3 



26 ON THE ATTRACTION OF GRAVITY. 

ion with each other. Weight is an effect of the power 
of attraction, without which the table and the book would 
have no weight whatever. 

Emily, I think I understand you ; is it not the at- 
traction of gravity, which makes bodies heavy ? 

Mrs. B. You are right. I told you that the attrac- 
tion of gravity was proportioned to the quantity of matter 
which bodies contained : now the earth consisting of a 
much greater quantity of matter than any body upon its 
surface, the force of its attraction must necessarily be 
greatest, and must draw every thing towards it ; in con- 
sequence of which, bodies that are unsupported fall to the 
ground, whilst those that are supported press upon the 
object which prevents their fall, with a weight equal to 
the force with which they gravitate towards the earth. 

Caroline. The same cause then which occasions the 
fall of bodies produces also their weight. It was very 
dull in me not to understand this before, as it is the na- 
tural and necessary consequence of attraction ; but the 
idea that bodies were not really heavy of themselves ap- 
peared to me quite incomprehensible. But, Mrs. B., if 
attraction is a property essential to matter, weight must 
be so likewise ; for how can one exist without the other ? 

Mrs. B. Suppose there were but one body existing in 
universal space, what would its weight be 1 

Caroline. That would depend upon its size ; or, more 
accurately speaking, upon the quantity of matter it con- 
tained. 

Emily. No, no ; the body would have no weight, 
whatever were its size ; because nothing would attract it. 
Am I not right, Mrs. B.? 

Mrs. B. You are : you must allow, therefore, that it 
would be possible for attraction to exist without weight ; 
for each of the particles of which the body was composed, 
would possess the power of attraction ; but they could 
exert it only amongst themselves ; the whole mass, hav- 
ing nothing to attract, or to be attracted by, would have 
no weight. 

Caroline. I am now well satisfied that weight is not 
•itial to the existence of bodies ; but what have you 

70. What is weight, or of what is it the effect? 71. If 

vere but one body in the universe, would there be any such 

thing as weight? 72. Can cohesive attraction exist where 

ilicrc j M-> \- eight ? 



ON THE ATTRACTION OF GRAVITY. 27 

to object to colours, Mrs. B. 1 You will not, I think, deny 
that they really exist in the bodies themselves. 

Mrs. B. When we come to treat of the subject of co- 
lours, I trust that I shall be able to convince you, that co- 
lours are likewise accidental qualities, quite distinct from 
the bodies to which they appear to belong. 

Caroline. Oh do pray explain it to us now, I am so 
very curious to know how that is possible. 

Mrs. B. Unless we proceed with some degree of or- 
der and method, you will in the end find yourself but lit- 
tle the wiser for all you learn. Let us therefore go on 
regularly, and make ourselves well acquainted with the 
general properties of bodies, before we proceed further. 

Emily. To return, then, to attraction, (which appears 
to me by far the most interesting of them, since it belongs 
equally to all kinds of matter,) it must be mutual between 
two bodies ; and if so, when a stone falls to the earth, the 
earth should rise part of the way to meet the stone 1 

Mrs. B. Certainly ; but you must recollect that the 
force of attraction is proportioned to the quantity of mat- 
ter which bodies contain, and if you consider the differ- 
ence there is in that respect, between a stone and the 
earth, you will not be surprised that you do not perceive 
the earth rise to meet the stone ; for though it is true that a 
mutual attraction takes place between the earth and the 
stone, that of the latter is so very small in comparison to 
to that of the former, as to render its effect insensible. 

Emily. But since attraction is proportioned to the 
quantity of matter which bodies contain, why do not the 
hills attract the houses and churches towards them 1 

Caroline. You surprise me, Emily ; what an idea ! 
How can the houses and churches be moved, when they 
are so firmly fixed in the ground ? 

Mrs. B. Emily's question is not absurd, and your 
answer, Caroline, is perfectly just ; but can you tell us 
why the houses and churches are so firmly fixed in the 
ground. 

Caroline. I am afraid I have answered right by mere 
chance ; for I begin to suspect that bricklayers and car- 
penters could give but little stability to their buildings, 
without the aid of attraction. 

73. If the attraction of gravitation is mutual between bodies, 
why do we not see the earth rise part way to meet the stone 
which is falling towards it ? 



28 ON THE ATTRACTION OF GRAVITY. 

Mrs. B. It is certainly the cohesive attraction between 
the bricks and the mortar which enables them to build 
walls, and these are so strongly attracted by the earth, as 
to resist every other impulse ; otherwise they would ne- 
cessarily move towards the hills and the mountains ; but 
the lesser force must yield to the greater. There are, how- 
ever, some circumstances in which the attraction of a large 
body has sensibly counteracted that of the earth. If, 
whilst standing on the declivity of a mountain, you hold a 
plumb-line in your hand, the weight will not fall perpen- 
dicular to the earth, but incline a little towards the moun- 
tain ; and this is owing to the lateral, or sideways attrac- 
tion of the mountain, interfering with the perpendicular 
attraction of the earth. 

Emily. But the size of a mountain is very trifling 
compared to the whole earth 1 

Mrs. B. Attraction, you must recollect, diminishes 
with distance ; and in the example of the plumb-line, the 
weight suspended is considerably nearer to the mountain 
than to the centre of the earth ? 

Caroline, Pray, Mrs. B., do the two scales of a ba- 
lance hang parallel to each other 1 

Mrs. B. You mean, I suppose, in other words, to in- 
quire whether two lines which are perpendicular to the 
earth, are parallel to each other ? I believe I guess the 
reason of your question ; but I wish you would endeavour 
to answer it without my assistance. 

Caroline. I was thinking that such lines must both 
tend by gravity to the same point, the centre of the earth ; 
now lines tending to the same point cannot be parallel, as 
parallel lines are always at an equal distance from each 
other, and would never meet. 

Mrs. B. Very well explained ; you see now the use 
of your knowledge of parallel lines : had you been igno- 
rant of their properties, you could not have drawn such 
a conclusion. This may enable you to form an idea of 
the great advantage to be derived even from a slight 
knowledge of geometry in the study of natural philoso- 
phy ; and if, after I have made you acquainted with the 
first elements, you should be tempted to pursue the study, 

74. And why are not houses and other objects at the side of a 
mountain attracted or drawn away from their foundations towards 
it ? 75. How can it be shown that mountains possess a side- 
ways attraction ? 76. Would two lines suspended by weights 

be parallel to each other ? 






ON THE ATTRACTION OF GRAVITY. 29 

I would advise you to prepare yourselves by acquiring 
some knowledge of geometry. This science would teach 
you that lines which fall perpendicular to the surface of a 
sphere cannot be parallel, because they would all meet, if 
prolonged to the centre of the sphere ; while lines that 
fall perpendicular to a plane or flat surface, are always 
parallel, because, if prolonged, they would never meet. 

Emily. And yet a pair of scales, hanging perpendicu- 
lar to the earth, appear parallel ? 

Mrs. B. Because the sphere is so large, and the scales 
consequently converge so little, that their inclination is 
not perceptible to our senses ; if we could construct a 
pair of scales whose beam would extend several degrees, 
their convergence would be very obvious ; but as this 
cannot be accomplished, let us draw a small figure of the 
earth, and then we may make a pair of scales of the pro- 
portion we please, (fig. 1. plate 1.) 

Caroline. This figure renders it very clear : then two 
bodies cannot fall to the earth in parallel lines ? 

Mrs. B. Never. 

Caroline. The reason that a heavy body falls quicker 
than a light one, is, I suppose, because the earth attracts 
it more strongly 1 

Mrs. B. The earth, it is true, attracts a heavy body 
more than a light one ; but that would not make the one 
fall quicker than the other. 

Caroline. Yet since it is attraction that occasions the 
fall of bodies, surely the more a body is attracted, the 
more rapidly it will fall. Besides, experience proves it to 
be so. Do we not every day see heavy bodies fall quickly, 
and light bodies slowly 1 

Emily. It strikes me, as it does Caroline, that as at- 
traction is proportioned to the quantity of matter, the 
earth must necessarily attract a body which contains a 
great quantity more strongly, and therefore bring it to the 
ground sooner than one consisting of a smaller quantity. 

Mrs. B. You must consider, that if heavy bodies are 
attracted more strongly than light ones, they require 
more attraction to make them fall, Remember that bo- 

77. Why would they not be ? 78. Why is not their con- 

vergency perceptible ?~ 79. What figure illustrates the con- 

vergency of two lines suspended perpendicularly to the surface of 

the earth ? 80. Do heavy and light bodies fall to the ground 

with equal rapidity P 
3* 



30 ON THE ATTRACTION OF URAVITV. 

dies have no natural tendency to fall, any more than to 
rise, or to move laterally, and that they will not fall un- 
it ss impelled by some force ; now this force must be pro- 
portioned to the quantity of matter it has to move : a 
body consisting of 1000 particles of matter, for instance, 
requires ten times as much attraction to bring it to the 
ground in the same space of time as a body consisting of 
only 100 particles. 

Caroline. I do not understand that ; for it seems to 
me that the heavier a body is, the more easily and rea- 
dily it fall?. 

Emily. I think I now comprehend it ; let me try if I 
can explain it to Caroline. Suppose that I draw towards 
me two weighty bodies, the one of 1001bs., the other of 
10001bs., must I not exert ten times as much strength to 
draw the larger one to me, in the same space of time as 
is required for the smaller one ? And if the earth draw a 
body of 10001bs., weight to it in the same space of time 
that it draws a body of lOOlbs., does it not follow that it 
attracts the body of lOOOlbs. weight with ten times the 
force that it does that of lOOlbs. 1 

Caroline. I comprehend your reasoning perfectly ; but 
if it were so, the body of lOOOlbs. weight, and that of lOOlbs. 
would fall with the same rapidity ; and the consequence 
would be, that all bodies, whether light or heavy, being at 
an equal distance from the ground, would fall to it in the 
same space of time : now it is very evident that this con 
elusion is absurd ; experience every instant contradicts it : 
observe how much sooner this book reaches the floor than 
this sheet of paper, when I let them drop together. 

Emily. That is an objection I cannot answer. I must 
refer it to you, Mrs. B. 

Mrs. B. I trust that we shall not find it insurmount- 
able. It is true that, according to the laws of attraction, 
all bodies at an equal distance from the earth, should fall 
to it in the same space of time ; and this would actually 
take place if no obstacle intervened to impede their fall. 
But bodies fall through the air, and it is the resistance 
of the air which makes bodies of different density 
fall with different degrees of velocity. They must 

w 1 To what must the force of gravity be proportional neces- 
sary in causing bodies of different weights to fall to the ground ? 

H2. What are the laws of attraction in regard to the falling 

of bodies at equal distances from the earth ? 83. But why then 

do heavy bodies fall quicker than light ones ? 



ON THE ATTRACTION OF GRAVITY. 31 

all force their way through the air, but dense heavy 
bodies overcome this obstacle more easily than rarer 
and lighter ones. 

The resistance which the air opposes to the fall of bo- 
dies is proportioned to their surface, not to their weight ; 
the air being inert, cannot exert a greater force to support 
the weight of a cannon-ball, than it does to support the 
weight of a ball (of the same size) made of leather ; but 
the cannon-ball will overcome this resistance more easily, 
and fall to the ground, consequently, quicker than the 
leather ball. 

Caroline. This is very clear, and solves the difficulty 
perfectly. The air offers the same resistance to a bit of 
lead and a bit of feather of the same size ; yet the one 
seems to meet with no obstruction in its fall, whilst the 
other is evidently resisted and supported for some time by 
the air. 

Emily. The larger the surface of a body, then, the 
more aifit covers, and the greater is the resistance it 
meets with from it. 

Mrs. B. Certainly : observe the manner in which 
this sheet of paper falls ; it floats awhile in the air, and 
then gently descends to the ground. I will roll the same 
piece of paper up into a ball : it offers now but a small 
surface to the air, and encounters therefore but little re- 
sistance : see how much more rapidly it falls. 

The heaviest bodies may be made to float awhile in the air, 
by making the extent of their surface counterbalance their 
weight. Here is some gold, which is the most dense body 
we are acquainted with, but it has been beaten into a very 
thin leaf, and offers so great an extent of surface in propor- 
tion to its weight, that its fall, you see, is still more retarded 
by the resistance of the air than that of the sheet of paper. 

Caroline. That is very curious ; and it is, I suppose, 
upon the same principle that iron boats may be made to 
float on water 1 

But, Mrs. B., if the air is a real body, is it not also 
subjected to the laws of gravity 1 

Mrs. B. Undoubtedly. 

Caroline. Then why does it not, like all other bodies, 
fall to the ground 1 

84. To what is the resistance, that the air opposes to falling 

bodies, proportioned ? 85. How can heavy bodies be made to 

float awhile in the air instead of falling immediately to the ground ? 
=86. Does the fiir gravitate towards the earth ? 



32 ON THE ATTRACTION OP GRAVITY. 

Mrs. B. On account of its spring or elasticity. The 
air is an elastick fluid j a species of bodies, the peculiar 
property of which is to resume, after compression, their ori- 
ginal dimensions ; and you must consider the air of which 
the atmosphere is composed as existing in a state of com- 
pression, for its particles being drawn towards the earth 
by gravity, are brought closer together than they would 
otherwise be, but the spring or elasticity of the air by which 
it endeavours to resist compression gives it a constant ten- 
dency to expand itself, so as to resume the dimensions it 
would naturally have, if not under the influence of gravity. 
The air may therefore be said constantly to struggle with 
the power of gravity without being able to overcome it. 
Gravity thus confines the air to the regions of our globe, 
whilst its elasticity prevents it from falling like other bo- 
dies to the ground. 

Emily. The air then is, I suppose, thicker, or I 
should rather say more dense, near the surface of the earth, 
than in the higher regions of the atmosphere ; for that part 
of the air which is nearer the surface of the earth must be 
most strongly attracted. 

Mrs. B. The diminution of the force of gravity, at so 
small a distance as that to which the atmosphere extends 
(compared with the size of the earth) is so inconsiderable 
as to be scarcely sensible ; but the pressure of the upper 
parts of the atmosphere on those beneath, renders the air 
near the surface of the earth much more dense than the 
upper legions. 

The pressure of the atmosphere has been compared to 
that of a pile of fleeces of wool, in which the lower fleeces 
are pressed together by the weight of those above ; these 
lie light and loose, in proportion as they approach the up- 
permost fleece, which receives no external pressure, and 
is confined merely by the force of its own gravity. 

Caroline. It has just occurred to me that there are some 
bodies which do not gravitate towards the earth. Smoke 
and steam, for instance, rise instead of falling. 

87. Why then does it not fall like other bodies completely to 

the surface of the earth ? 88. What two forces continually 

operate against each other on the air ? 89. Is the air of the 

same density at the surface of the earth as at a distance from it ? 

90. At which is the density the greatest ? 91. Why is the 

air more dense at the surface of the earth than at a distance from 

it ? 92. To what has the pressure of the atmosphere been 

compared ? 



ON THE ATTRACTION OF GRAVITY. 33 

Mrs. B. It is still gravity which produces their as- 
cent ; at least, were that power destroyed, these bodies 
would not rise. 

Caroline. I shall be out of conceit with gravity, if it 
is so inconsistent in its operations. 

Mrs. B. There is no difficulty in reconciling this ap- 
parent inconsistency of effect. The air near the earth is 
heavier than smoke, steam, or other vapours ; it conse- 
quently not only supports these light bodies, but forces 
them to rise, till they reach a part of the atmosphere, the 
weight of which is not greater than their own, and then 
they remain stationary. Look at this basin of water : 
why does the piece of paper which I throw into it float 
on the surface 1 

Emily. Because, being lighter than the water, it is 
supported by it. 

Mrs. B. And now that I pour more water into the 
basin, why does the paper rise ? 

Emily. The water being heavier than the paper, gets 
j?eneath it and obliges it to rise. 

Mrs. B. In a similar manner are smoke and vapour 
forced upwards by the air ; but these bodies do not, like 
the paper, ascend to the surface of the fluid, because, as we 
observed before, the air being thinner and lighter as it is 
more distant from the earth,vapours rise only till they attain 
a region of air of their own density. Smoke , indeed, ascends 
but a very little way ; it consists of minute particles of fuel 
carried up by a current of heated air from the fire below : 
heat, you recollect, expands all bodies ; it consequently ra- 
refies air, and renders it lighter than the colder air of the 
atmosphere ; the heated air from the fire carries up with it 
vapour and small particles of the combustible materials 
which are burning in the fire. When this current of hot air 
is cooled by mixing with that of the atmosphere, the minute 
particles of coal or other combustible fall, and it is this 
which produces the small black flakes which render the air 
and every thing in contact with it, in London, so dirty. 

Caroline. You must, however, allow me to make one 
more objection to the universal gravity of bodies ; which 

93. How does gravity operate in causing smoke and steam to 

rise instead of falling to the earth ? 94. How high will they 

rise before they become stationary ? 95. What familiar illus- 
tration is given of the principle upon which smoke and vapour 
ascend ? 96. Of what does smoke consist ? 



34 ON THE ATTRACTION OF GRAVITY. 

is the ascent of air balloons, the materials of which are 
undoubtedly heavier than air : how, therefore, can they 
be supported by it ? 

Mrs* B. I admit that the materials of which balloons 
are made are heavier than the air ; but the air with which 
they are filled is an elastick fluid, of a different nature from 
the atmospherick air, and considerably lighter ; so that on 
the whole, the balloon is lighter than the air which it dis- 
places, and consequently will rise, on the same principle as 
smoke and vapour. Now, Emily, let me hear if you can 
explain how the gravity of bodies is modified by the effect 
of the air ? 

Emily. The air forces bodies which are lighter than 
itself to ascend ; those that are of an equal weight will 
remain stationary in it ; and those that are heavier will 
descend through it ; but the air will have some effect on 
these last ; for if they are not much heavier, they will with 
difficulty overcome the resistance they meet with in pass- 
ing through it, they will be borne up by it, and their fall 
will be more or less retarded. 

Mrs. B. Very well. Observe how slowly this light feather 
falls to the ground, while a heavier body, like this marble, 
overcomes the resistance which the air makes to its descent 
much more easily, and its fall is proportionally more rapid. 
t now throw a pebble into this tub of water ; it does not reach 
the bottom near so soon as if there were no water in the tub, 
because it meets with resistance from the water. Suppose 
that we could empty the tub, not only of water, but of air also, 
the pebble would then fall quicker still, as it would in that 
case meet with no resistance at all to counteract its gravity. 

Thus you see that it is not the different degrees of 
gravity, but the resistance of the air, which prevents bo- 
dies of different weight from falling with equal velocities ; 
if the air did not bear up the feather, it would reach the 
ground as soon as the marble. 

Caroline. I make no doubt that it is so ; and yet I do 
not feel quite satisfied. I wish there were some place 
void of air, in which the experiment could be made. 

Mrs. B. If that proof will satisfy your doubts, I can 
give it you. Here is a machine called an air pump, (fig. 2. 
pi. I.) by means of which the air may be expelled from 

\)7. On what principle does a balloon rise, since it \s made of 

materials heavier than the air through which it rises? 98 

How is the (rrnvity f bodies modified by the effect of the air? 
— — i) 1 .) What is the use of the air pump ? 



ON THE ATTRACTION OF GRAVITY. 85 

any close vessel which is placed over this opening, through 
which the air is pumped out. Glasses of various shapes, 
usually called receivers, are employed for this purpose. 
We shall now exhaust the air from this tall receiver which 
is placed over the opening, and we shall find that bodies 
of whatever weight or size within it, will fall from the top 
to the bottom in the same space of time. 

Caroline. Oh, I shall be delighted with this experi- 
ment ; what a curious machine ! how can you put the 
two bodies of different weight within the glass, without 
admitting the air ? 

Mrs. B. A guinea and a feather are already placed 
there for the purpose of the experiment : here is, you see, 
a contrivance to fasten them in the upper part of the glass ; 
as soon as the air is pumped out, I shall turn this little 
screw, by which means the brass plates which support 
them will be inclined, and the two bodies will fall. — Now 
I believe I have pretty well exhausted the air. 

Caroline. Pray let me turn the screw. I declare, 
they both reached the bottom at the same instant ! Did 
you see, Emily, the feather appeared as heavy as the 
guinea ? 

Emily. Exactly ; and fell just as quickly. How won- 
derful this is ! what a number of entertaining experi- 
ments might be made with this machine ! 

Mrs. B. No doubt there are a great many ; but we 
shall reserve them to elucidate the subjects to which 
they relate ; if I had not explained to you why the guinea 
and the feather fell with equal velocity, you would not 
have been so well pleased with the experiment. 

Emily. I should have been as much surprised, but not 
so much interested ; besides, experiments help to imprint 
on the memory the facts they are intended to illustrate ; 
it will be better therefore for us to restrain our curiosity, 
and wait for other experiments in their proper places. 

Caroline. Pray by what me?ns is the air exhausted in 
this receiver 1 

Mrs. B. You must learn something of mechanicks in 
order to understand the construction of a pump. At our 
next meeting, therefore, I shall endeavour to make you 
acquainted with the laws of motion, as an introduction to 
that subject. 

100. Can a feather be placed in a situation to fall as quickly as 
a stone ? 101. In what manner can it be done ? 



36 ON THE LAWS OF MOTION. 

CONVERSATION III. 

ON THE LAWS OF MOTION. 

On Motion ; Of the Inertia of Bodies ; Of Force to 
produce Motion ; Direction of Motion ; Velocity, Ah- 
solute and Relative ; Uniform Motion ; Retarded Mo- 
tion; Accelerated Motion; Velocity of Falling Bo- 
dies; Momentum; Action and Re-action Equal; 
Elasticity of Bodies ; Porosity of Bodies ; Reflected 
Motion ; Angles of Incidence and Reflection. 

MRS. B. 

The science of mechanicks is founded on the laws of 
motion ; it will, therefore, be necessary to make you ac- 
quainted with these laws before we examine the mecha- 
nical powers. Tell me, Caroline, what do you understand 
by the word motion 1 

Caroline. I think I understand it perfectly, though I 
am at a loss to describe it. Motion is the act of moving 
about, going from one place to another ; it is the contrary 
of remaining at rest. 

Mrs. B. Very well. Motion then consists in a change 
of place ; a body is in motion whenever it is changing its 
situation with regard to a fixed point. 

Now since we have observed that one of the general 
properties of bodies is Inertia, that is, an entire passiveness 
either with regard to motion or rest, it follows that a body 
cannot move "ithout being put into motion ; the power 
which puts a body into motion is called force ; thus, the 
stroke of the hammer is the force which drives the nail ; 
the pulling of the horse that which draws the carriage, 
&c. Force then is the cause which produces motion. 

Emily. And may we not say that gravity is the force 
which occasions the fall of bodies ? 

Mrs. B. Undoubtedly. I had given you the most fa- 
miliar illustrations in order to render the explanation 
clear ; but since you seek for more scientifick examples, 
you may say that cohesion is the force which binds the 
particles of bodies together, and heat that which drives 
them asunder. 

102. On what is the science of mechanicks founded ? 103. 

What is to be understood by the term motion ? 104. What ih 

the power called that puts a body in motion ? 



ON THE LAWS OP MOTION. 37 

The motion of a body acted upon by a single force is 
always in a straight line, in the direction in which it re- 
ceived the impulse. 

Caroline. That is very natural ; for as the body is in- 
ert, and can move only because it is impelled, it will move 
only in the direction in which it is impelled. The degree 
of quickness with which it moves, must, I suppose, also de- 
pend upon the degree of force with which it is impelled. 

Mrs. B. Yes ; the rate at which a body moves, or the 
shortness of the time which it takes to move from one 
place to another, is called its velocity ; and it is one of 
the laws of motion that the velocity of the moving body is 
proportional to the force by which it is put in motion. 

We must distinguish between absolute and relative ve- 
locity. 

The velocity of a body is called absolute^ if we consider 
the motion of the body in space, without any reference to 
that of other bodies. When for instance a horse goes fifty 
miles in ten hours, his velocity is five miles an hour. 

The velocity of a body is termed relative^ when com- 
pared with that of another body which is itself in motion. 
For instance, if one man walks at the rate of a mile an 
hour, and another at the rate of two miles an hour, the 
relative velocity of the latter is double that of the former, 
but the absolute velocity of the one is one mile, and that 
of the other two miles an hour. 

Emily. Let me see if I understand it. The relative 
velocity of a body is the degree of repidity of its motion 
compared with that of another body ; thus, if one ship 
sail three times as far as another ship in the same space 
of time, the velocity of the former is equal to three times 
that of the latter. 

Mrs. B. The general rule may be expressed thus . 
the velocity of a body is measured by the space over 
which it moves, divided by the time which it employs in 
that motion : thus if you travel one hundred miles in 
twenty hours, what is your velocity in each hour 1 

105. In what direction is the motion of a body acted on by a 
single force ? 106. What is meant by the velocity of motion ? 

107. To what is the velocity of a moving body proportional ? 

108. What is called absolute velocity ?— 109. When is the 

velocity of a moving body called relative ? 110. What would 

be instances of relative velocity ? 111. What is the general 

rule for calculating the velocity of a moving body ? 

4 



38 ON THE LAWS OF MOTION. 

Emily. I must divide the space, which is one hundred 
miles, by the time, wfiich is twenty hours, and the answer 
will be five miles an hour. Then, Mrs. B., may we not 
reverse this rule and say, that the time is equal to the 
space divided by the velocity ; since the space one hun- 
dred miles, divided by the velocity five miles, gives twen- 
ty hours for the time ? 

Mrs. B. Certainly ; and we may say also that space 
is equal to the velocity multiplied by the time. Can you 
tell me, Caroline, how many miles you will have travelled, 
if your velocity is three miles an hour, and you travel six 
hours 1 J 

Caroline. Eighteen miles ; for the product of 3 mul- 
tiplied by 6, is 18. 

Mrs. B. Xauppose that you understand what is meant 
by the terms uniform, accelerated, and retarded motion. 

Emily. I conceive uniform motion to be that of a body 
whose motion is regular, and at an equal rate throughout ; 
for instance, a horse that goes an equal number of miles 
every hour. But the hand of a watch is a much better 
example, as its motion is so regular as to indicate the time. 

Mrs. B. You have a right idea of uniform motion ; 
but it would be more correctly expressed by saying, that 
the motion of a body is uniform when it passes over equal 
spaces in equal times. Uniform motion is produced by 
a force having acted on a body once, and having ceased 
to act ; as for instance, the stroke of a bat on a cricket 
ball. 

Caroline. But the motion of a cricket ball is not uni- 
form ; its velocity gradually diminishes till it falls to the 
ground. 

Mrs. B. Recollect that the cricket ball is inert, and 
has no more power to stop than to put itself in motion ; if 
it falls, therefore, it must be stopped by some force supe- 
riour to that by which it was projected, and which destroys 
its motion. 

Caroline. And it is no doubt the force of gravity which 
counteracts and destroys that of projection ; but if there 
were no such power as gravity, would the cricket ball 
never stop ? 

Mrs. B. If neither gravity nor any other force, such 
as the resistance of the air, opposed its motion, the cricket 

112. When is the motion of a body termed uniform ? 113. 

How is uniform motion produced ? 



ON THE LAWS OF MOTION. 39 

ball, or even a stone thrown by the hand, would proceed 
onwards in a right line, and with a uniform velocity for 
ever. 

Caroline. You astonish me ! I thought that it was im- 
possible to produce perpetual motion ? 

Mrs. B. Perpetual motion cannot be produced by art, 
because gravity ultimately destroys all motion that hu- 
man powers can produce. 

Emily. But independently of the force of gravity, I 
cannot conceive that the little motion I am capable of 
giving to a stone would put it in motion for ever. 

Mrs. B. The quantity of motion you communicate to 
the stone would not influence its duration : if you threw 
it with little force it would move slowly ; for its velocity, 
you must remember, will be proportional to the? force with 
which it is projected ; but if there is nothing to obstruct its 
passage, it will continue to move with the same velocity, 
and in the same direction as when yoiurirst projected it. 

Caroline. This appears to me quite incomprehensible ; 
we do not meet with a single instance of it in nature. 

Mrs. B. I beg your pardon. When you come to 
study the motion of the celestial bodies, you will find that 
nature abounds with examples of perpetual motion ; and 
that it conduces as much to the harmony of the system of 
the universe as the prevalence of it would to the destruc- 
tion of all comfort on our globe. The wisdom of Provi- 
dence has therefore ordained insurmountable obstacles to 
perpetual motion here below ; and though these obstacles 
often compel us to contend with great difficulties, yet 
there results from it that order, regularity, and repose, so 
essential to the preservation of all the various beings of 
which this world is composed. 

Now can you tell me what is retarded motion ? 

Caroline. Retarded motion is that of a body which 
moves every moment slower and slower : thus when I 
am tired with walking fast, I slacken my pace ; or when 
a stone is thrown upwards, its velocity is gradually di- 
minished by the power of gravity. 

Mrs. B. Retarded motion is produced by some force 
acting upon the body in a direction opposite to that which 
first put it in motion : you who are an animated being, 
endowed with power and will, may slacken your pace, or 

114. What is the reason that perpetual motion cannot be pro- 
duced ? 115. What is retarded motion ? - — 116. How is re- 
tarded motion produced ? 



40 ON THE LAWS OF MOTION. 

stop to rest when you are tired ; but inert matter is inca- 
pable of any feeling of fatigue, can never slacken itspa^e 
and never stop, unless retarded or arrested in its course 
by some opposing force ; and as it is the laws of inert 
todies which mechanicks treat of, I prefer your illustra- 
tion of the stone retarded in its ascent. Now, Emily, it 
is your turn ; what is accelerated motion ? 

Emily. Accelerated motion, I suppose, takes place 
when the velocity of a body is increased ; if you had not 
objected to our giving such active bodies as ourselves as 
examples, I should say that my motion is accelerated if I 
change my pace from walking to running. I cannot think 
of any instance of accelerated motion in inanimate bodies ; 
all motion of inert matter seems to be retarded by gravity. 

Mrs. B. Not in all cases ; for the power of gravitation 
sometimes produces accelerated motion ; for instance, a 
stone falling from a height moves with a regularly acce- 
lerated motion. 

Emily. True ; because the nearer it approaches the 
earth, the more it is attracted by it. 

Mrs. B. You have mistaken the cause of its accele- 
ration of motion ; for though it is true th?,t the force of 
gravity increases as a body approaches the earth, the dif- 
ference is so trifling at any small distance from its surface 
as not to be perceptible. 

Accelerated motion is produced when the force which 
put a body in motion continues to act upon it during 
its motion, so that its motion is continually increased. 
When a stone falls from a height, the impulse which it re- 
ceives from gravity during the first instant of its fall, would 
be sufficient to bring it to the ground with a uniform ve- 
locity : for, as we have observed, a body having been once 
acted upon by a force, will continue to move with a uni- 
form velocity ; but the stone is not acted upon by gravity 
merely at the first instant of its fall — this power continues 
to impel it during the whole of its descent, and it is this 
continued impulse which accelerates its motion. 

Emily. I do not quite undertand that. 

Mrs. B. Let us suppose that the instant after you 
have let fall a stone from a high tower, the force of gra- 
vity were annihilated, the body would nevertheless con- 

117. What is accelerated motion? 118. What is an in- 
stance of accelerated motion ? 119. How does gravity accele- 
rate the motion of falling bodies ? 



ON THE LAWS OF MOTION. 41 

tinue to move downwards, for it would have received a 
first impulse from gravity, and a body once put in motion 
will not stop unless it meets with some obstacle to impede 
its course ; in this case its velocity would be uniform, for 
though there would be no obstacle to obstruct its descent, 
there would be no force to accelerate it. 

Emily. That is very clear. 

Mrs. B. Then you have only to add the power of 
gravity constantly acting on the stone d.iring its descent, 
and it will not be difficult to understand that its motion 
will become accelerated, since the gravity which acts on 
the stone during the first instant of its descent, will con- 
tinue in force every instant till it reaches the ground. 
Let us suppose that the impulse given by gravity to the 
stone during the first instant of its descent oe equal to one, 
the next instant we shafl find that an additional impulse 
gives the stone an additional velocity equal to one, so 
that the accumulated velocity is now equal to two ; the 
following instant another impulse increases the velocity to 
three, and so on till the stone reaches the ground. 

Caroline. Now I understand it ; the effects of preced 
ing impulses must be added to the subsequent velocities. 

Mrs. B. Yes ; it has been ascertained both by expe- 
riment and calculations, which it would be too difficult for 
us to enter into, that heavy bodies descending from a height 
by the force of gravity, fall sixteen feet the first second of 
time, three times that distance in the next, five times in the 
third second, seven times in the fourth, and so on, regu- 
larly increasing their velocities according to the number 
of seconds during which the body has been falling. 

Emily. If you throw a stone perpendicularly upwards, 
is it not the same length of time ascending that it is de- 
scending ? 

Mrs. B. Exactly ; in ascending, the velocity is di- 
minished by the force of gravity ; in descending, it is ac- 
celerated by it. 

Caroline. 1 should then have imagined that it would 
have fallen quicker than it rose 1 

Mrs. B. You must recollect that the force with which 
it is projected must be taken into the account ; and that 

120. What distance will a heavy body, suspended in the air, 
fall the first second of time ? What distance the second ? What 
the third ? 121. How does the time of an ascending body al- 
ways compare with the time of its descent ? 

4* 



13 ON THE LAWS OF MOTION 

this force is overcome and destroyed by gravity before the 
body falls. 

Caroline. But the force of projection given to a stone 
in throwing it upwards, cannot always be equal to the 
force of gravity in bringing it down again, for the force 
of gravity is always the same, whilst the degree of im- 
pulse given to the stone is optional ; I may throw it up 
gently or with violence. 

Mrs, B. If you throw it gently, it will not rise high; 
perhaps only sixteen feet, in which case it will fall in one 
second of time. Now it is proved by experiment, that an 
impulse requisite to project a body sixteen feet upwards, 
will make it ascend that height in one second ; here then 
the times of the ascent and descent are equal. But sup- 
posing it be required to throw a stone twice that height, 
the force must be proportionally greater. 

You see then, that the impulse of projection in throw- 
ing a body upwards, is always equal to the action of. the 
force of gravity during its descent ; and that it is the 
greater or less distance to which the body rises, that 
makes these two forces balance each other. 

I must now explain to you what is meant by the mo- 
mentum of bodies. It is the force, or power, with which 
a body in motion, strikes against another body. The 
momentum of a body is composed of its quantity of 
matter, multiplied by its quantity of motion ; in other 
words its weight and its velocity. 

Caroline. The quicker a body moves, the greater, no 
doubt, must be the force with which it would strike against 
another body. 

Emily. Therefore a small body may have a greater mo- 
mentum than a large one, provided its velocity be sufficient- 
ly greater ; for instance, the momentum of an arrow shot 
from a bow must be greater than a stone thrown by the hand. 

Caroline. We know also by experience, that the 
heavier a body is, the greater is its force ; it is not there- 
fore difficult to understand, that the whole power or mo- 
mentum of a body must be composed of these two pro- 
perties ; but I do not understand, why they should 

122. To what is the impulse of projection, in throwing a body 

upwards, equal ? 123- What is the momentum of a body ? 

124. Of what is the momentum of a body composed ? 

196. In what way can a smaller body have a greater momentum 

than a lar^or body ' 



ON THE LAWS OF MOTION. 43 

be multiplied, the one by the other ; I should have sup- 
posed that the quantity of matter should have been added 
to the quantity of motion ? 

Mrs. B. It is found by experiment, that if the weight 
of a body is represented by the number 3, and its velocity 
also by 3, its momentum will be represented by 9 ; not 6, 
as would be the case, were these figures added, instead of 
being multiplied together. I recommend it to you to be 
careful to remember the definition of the momentum of 
bodies, as it is one of the most important points in mecha- 
nicks ; you will find, that it is from opposing motion to 
matter, that machines derive their powers.* 

The re-action of bodies is the next law of motion which 
I must explain to you. When a body in motion strikes 
against another body, it meets with resistance from it ; 
the resistance of the body at rest will be equal to the 
blow struck by the body in motion ; or to express myself 
in philosophical language, action and re-action will be 
equal, and in opposite directions. 

Caroline. Do you mean to say, that the action of the 
body which strikes, is returned with equal force by the 
body which receives the blow 1 

Mrs. B. Exactly. 

Caroline. But if a man strikes another on the face with 
his fist, he surely does not receive as much pain by the 
re-action as he inflicts by the blow ? 

Mrs. B. No ; but this is simply owing to the knuckles 
having much less feeling than the face. 

Here are two ivory balls suspended by threads, (plate 
1. fig. 3.) draw one of them, A, a little on one side, — now 
let it go ; — it strikes you see against the other ball B, and 
drives it off, to a distance equal to that through which the 

* In comparing together the momenta of different bodies, we 
must be attentive to measure their weights and velocities, by the 
same denomination of weights and of spaces, otherwise the results 
would not agree. Thus if we estimate the weight of one body in 
ounces, we must estimate the weight of the rest also in ounces, 
and not in pounds ; and in computing the velocities, in like man- 
ner, we should adhere to the same standard of measure, both of 
space and of time ; as for instance, the number of feet in one se- 
cond, or of miles in one hour. 

126. If the weight of a body be respresented by 3, and its ve- 
locity by 3, what will be its momentum ? 127. What is 

meant by the term re-action, in mechanicks ? 128. To what is 

re-action equal ? ~129 f What does figure 3. Plate J. illustrate ? 



44 ON THE LAWS OF MOTIOfc. 

first ball fell ; but the motion of A is stopped, because 
when it struck B, it received in return a blow equal to 
that it gave, and its motion was consequently destroyed. 

Emily. I should have supposed that the motion of the 
ball A was destroyed, because it had communicated all 
its motion to B. 

Mrs, B. It is perfectly true, that when one body 
strikes against another, the quantity of motion communi- 
cated to the second body, is lost by the first ; but this loss 
proceeds from the action of the body which is struck. 

Here are six ivory balls hanging in a row, (fig. 4.) draw 
the first out of the perpendicular, and let it fall against 
the second. None of the balls appear to move, you see, 
except the last, which flies off as far as the first ball fell ; 
can you explain this ? 

Caroline. I believe so. When the first ball struck the 
second, it received a blow in return, which destroyed its 
motion ; the second ball, though it did not appear to move, 
must have struck against the third; the re-action of which 
set it at rest ; the action of the third ball must have been 
destroyed by the re-action of the fourth, and so on till mo- 
tion was communicated to the last ball, which, not being 
re-acted upon, flies off. 

Mrs. B. Very well explained. Observe, that it is 
only when bodies are elastick, as these ivory balls are, that 
the stroke returned is equal to the stroke given. I will 
show you the difference with these two balls of clay, (fig. 
5.) which are not elastick ; when you raise one of these, 

D, out of the perpendicular, and let it fall against the other, 

E, the re-action of the latter, on account of its not being 
elastick, is not sufficient to destroy the motion of the for- 
mer ; only part of the motion of D will be communicated to 
E, and the two balls will move on together to d and e which 
is not so great a distance as that through which D fell. 

Observe how useful re-action is in nature. Birds in fly- 
ing strike the air with their wings, and it is the re-action of 
the air which enables them to rise, or advance forwards ; 
re-action being always in a contrary direction to action. 

130. How would you explain the operation of action and re- 
action, as illustrated by the six ivory balls in Figure 4, Plate I. : 

131. Is the re-action of all bodies equal to the action when 

a blow is given ? 132. In what ones is it equal? 133. 

What is the object of figure 5, Plate I. ? 134. How does this 

figure show that the re-action of non-elastick bodies is not equal 

if» the action ? 135. On what mechanical principle is it that 

birds are able to fly. 



ON THE LAWS OF MOTION. 45 

Caroline, I thought that birds might be lighter than 
the air, when their wings were expanded, and by that 
means enabled to fly. 

Mrs. B. When their wings are spread, they are bet- 
ter supported by the air, as they cover a greater extent of 
surface ; but they are still much too heavy to remain in that 
situation, without continually flapping their wings, as you 
may have noticed, when birds hover over their nests : the 
force with which their wings strike against the air must 
equal the weight of their bodies, in order that the re-action 
of the air may be able to support that weight ; the bird will 
then remain stationary. If the stroke of the wings be 
greater than is required merely to support the bird, the 
re-action of the air will make it rise ; if it be less, it will 
gently descend ; and you may have observed the lark, 
sometimes remaining with its wings extended, but mo- 
tionless : in this state it drops rapidly into its nest. 

Caroline. What a beautiful etfect this is of the law of 
re-action ! But if flying is merely a mechanical operation, 
Mrs. B., why should we not construct wings, adapted to 
the size of our bodies, fasten them to our shoulders, move 
them with our arms, and soar into the air. 

Mrs. B. Such an experiment has been repeatedly at- 
tempted, but never with success ; and it is now considered 
as totally impracticable. The muscular power of birds is 
greater in proportion to their weight than that of man ; 
were we therefore furnished with wings sufficiently large to 
enable us to fly, we should not have strength to put them in 
motion. In swimming, a similar action is produced on the 
water, as that on the air in flying ; and also in rowing ; you 
strike the water with the oars, in a direction opposite to that 
in which the boat is required to move : and it is the re-ac- 
tion of the water on the oars which drives the boat along. 

Emily. You said, that it was in elastick bodies only, 
that re-action was equal to action ; pray what bodies are 
elastick besides the air. 

Mrs. B. In speaking of the air, I think we defined 
elasticity to be a property, by means of which, bodies that 
are compressed returned to their former state. If I bend 

136. How must a bird strike the air with its wings so as to re- 
main stationary ? — So as to rise ? — So as to descend ? 137. 

If flying is only the effect of re-action, why could not a man be fur- 
nished with wings so as to fly ? 138. How is swimming effect- 
ed ? 139. On what principle is a boat moved upon the water ? 

140. What is to be understood by the elasticity of a body ? 



46 ON THE LAWS OF MOTION. 

this cane, as soon as I leave it at liberty it recovers its 
former position ; if I press my finger upon your arm, as 
soon as I remove it, the flesh, by virtue of its elasticity, rises 
and destroys the impression I made. Of all bodies, the 
air is the most eminent for this property, and it has thence 
obtained the name of elastick fluid. Hard bodies are in 
the next degree elastick : if two ivory, or metallic balls 
are struck together, the parts at which they touch will be 
flattened : but their elasticity will make them instanta- 
neously resume their former shape. 

Caroline. But when two ivory balls strike c gainst each 
other, as they constantly do on a billiard table, no mark 
or impression is made by the stroke. 

Mrs. B. I beg your pardon ; but you cannot perceive 
any mark, because their elasticity instantly destroys all 
trace of it. 

Soft bodies, which easily retain impression, such as clay, 
wax, tallow, butter, &c. have very little elasticity ; but of 
all descriptions of bodies liquids are the least elastick. 

Emily. If sealing-wax were elastick, instead of retain- 
ing the impression of a seal, it would resume a smooth 
surface as soon as the weight of the seal was removed. 
But pray what is it that produces the elasticity of bodies ? 

Mrs. B. There is great diversity of opinion upon 
that point, and I cannot pretend to decide which ap- 
proaches nearest to the truth. Elasticity implies suscep- 
tibility of compression, and the susceptibility of compres- 
sion depends upon the porosity of bodies ; for were there 
no pores or spaces between the particles of matter of which 
a body is composed, it could not be compressed. 

Caroline. That is to say, that if the particles of bodies 
were as close together as possible, they could not be 
squeezed closer. 

Emily. Bodies then, whose particles are most distant 
from each other, must be most susceptible of compression, 
and consequently most elastick ; and this you say is the 
case with air, which is perhaps the least dense of all bodies 1 

Mrs. B. You will not in general find this rule hold 
good, for liquids have scarcely any elasticity, whilst hard 
bodies are eminent for this property, though the latter are 
certainly of much greater density than the former; elas- 

141. What bodies are most distinguished for elasticity? 

142. What bodies are not elastick? 143. On what is elasti- 

city supposed to depend ? 



ON THE LAWS OF MOTION. 47 

ticity implies, therefore, not only a susceptibility of com- 
pression, but depends upon the power of resuming its for- 
mer state after compression. 

Caroline. But surely there can be no pores in ivory 
and metals, Mrs. B. ; how then can they be susceptible 
of compression ? 

Mrs. B. The pores of such bodies are invisible to the 
naked eye, but you must not thence conclude that they 
have none ; it is, on the contrary, well ascertained that 
gold, one of the most dense of all bodies, is extremely po- 
rous, and that these pores are sufficiently large to admit 
water when strongly compressed to pass through them. 

This was shown by a celebrated experiment made 
many years ago at Florence. 

Emily. If water can pass through gold, there must 
certainly be pores or interstices which afford it a passage ; 
and if gold is so porous, what must other bodies be which 
are so much less -dense than gold ! 

Mrs. B. The clrief difference in this respect is, I be- 
lieve, that the pores in some bodies are larger than in 
others ; in cork, sponge, and bread, they form considerable 
cavities ; in wood and stone, when not polished, they are 
generally perceptible to the naked eye ; whilst in ivory, me- 
tals, and all varnished and polished bodies, they cannot be 
discerned. To give you an idea of the extreme porosity of 
bodies, Sir Isaac Newton conjectured that if the earth were 
so compressed as to be absolutely without pores, its dimen- 
sions might possibly not be more than a cubic inch. 

Caroline. What an idea ! Were we not indebted to 
Sir Isaac Newton for the theory of attraction, I should be 
tempted to laugh at him for such a supposition. What 
insignificant little creatures we should be ! 

Mrs. B. If our consequence arose from the size of 
our bodies, we should indeed be but pigmies ; but remem- 
ber that the mind of Newton was not circumscribed by 
the dimensions of its envelope. 

Emily. It is, however, fortunate that heat keeps the 
pores of matter open and distended, and prevents the at- 
traction of cohesion from squeezing us into a nut-shell. 

Mrs. B. Let us now return to the subject of re-action, 
on which we have some further observations to make. 

144. Is it supposed that ivory balls, metals, and other hard sub- 
stances are porous ? 145. How has it been proved that gold 

is porous ? 146. What conjecture did Sir Isaac Newton form 

concerning the porosity of the earth ? 



48 ON THE LAWS OF MOTION. 

It is re-action, being contrary to action, which produces 
reflected motion. If you throw a ball against the wall, it 
rebounds ; this return of the ball is owing to the re-action 
of the wall against which it struck, and is called reflected 
motion. 

Emily. And I now understand why balls filled with 
air rebound better than those stuffed with bran and wool, 
air being most susceptible of compression and most elas- 
tick, the re-action is more complete. 

Caroline. I have observed that when I throw a ball 
straight against the wall, it returns straight to my hand ; 
but if I throw it obliquely upwards, it rebounds still higher, 
and I catch when it falls. 

Mrs. B. You should not say straight, but perpendi- 
cularly against the wall ; for straight is a general term for 
lines in all directions which are neither curved nor bent, 
and is therefore equally applicable to oblique or perpendi- 
cular lines. 

Caroline. I thought that perpendicularly meant either 
directly upwards or downwards. 

Mrs. B. In those directions lines are perpendicular 
to the earth. A perpendicular line has always a reference 
to something towards which it is perpendicular ; that is to 
say, that it inclines neither to the one side nor the other, but 
makes an equal angle on every side. Do you understand 
what an angle is ? 

Caroline. Yes, I believe so : it is two lines meeting in 
a point. 

Mrs. B. Well then, let the line A B (plate II, fig. 1,) re- 
present the floor of the room, and the line C D that in which 
you throw a ball against it : the line C D, you will observe, 
forms two angles with the line A B, and those two angles 
are equal. 

Emily. How can the angles be equal, while the lines 
which compose them are of unequal length ? 

Mrs. B. An angle is not measured by the length of 
the lines, but by their opening. 

Emily. Yet the longer the lines are, the greater is the 
opening between them. 

Mrs. B. Take a pair of compasses and draw a circle 
over these angles, making the angular point the centre. 

147. What is reflected motion ? 148. What produces it ? 

149. What is meant by a perpendicular line ? 150. What 

angle ? 151 What does Fig. 1. plate II. illustrate ?— ■ — 

152. By what is an angle measured ? 



ON THE LAWS OF MOTION. 49 

Emily. To what extent must I open the compasses 1 

Mrs. B. You may draw the circle what size you 
please, provided that it cuts the lines of the angles we 
are to measure. All circles, of whatever dimensions, are 
supposed to be divided into 360 equal parts, called de- 
grees; the opening of an angle, being therefore a portion 
of a circle, must contain a certain number of degrees ; 
the larger the angle, the greater the number of degrees, 
and the two angles are said to be equal when they con- 
tain an equal number of degrees. 

Emily. Now I understand it. As the dimensions of 
an angle depend upon the number of degrees contained 
between its lines, it is the opening and not the length of 
its lines, which determines the size of the angle. 

Mrs. B. Very well : now that you have a clear idea 
of the dimensions of angles, can you tell me how many 
degrees are contained in the two angles formed by one 
line falling perpendicular on another, as in the figure I 
have just drawn 1 

Emily. You must allow me to put one foot of the 
compasses at the point of the angles, and draw a circle 
round them, and then I think I shall be a,ble to answer 
your question : the two angles are together just equal to 
half a circle, they contain therefore 90 degrees each ; 90 
degrees being a quarter of 360. 

Mrs. B. An angle of 90 degrees is called a right 
angle, and when one line is perpendicular to another, it 
forms, you see, (fig. 1.) a right angle on either side. 
Angles containing more than 90 degrees are called obtuse 
angles (fig. 2 ;) and those containing less than 90 degrees 
are called acute angles, (fig. 3.) 

Caroline. The angles of this square table are right 
angles, but those of the octagon table are obtuse angles ; 
and the angles of sharp-pointed instruments are acute 
angles. 

Mrs. B. Very well. To return now to your obser- 
vation, that if a ball is thrown obliquely against the wall 
it will not rebound in the same direction ; tell me, have 
you ever played at billiards 1 

153. Into how many degrees are all circles divided ? 154 

When are two angles said to be equal ? 155. How many de- 
grees are contained in the two angles formed by the figure named ? 

156. What is called a right angle ? — An obtuse angle ? — An 

acute angle ? 

5 



50 ON THE LAWS OF MOTION. 

Caroline. Yes, frequently ; and I have observed that 
when I push the ball perpendicularly against the cushion, 
it returns in the same direction ; but when I send it ob- 
liquely to the cushion, it rebounds obliquely, but on the 
opposite side ; the ball in this latter case describes an 
angle, the point of which is at the cushion. I have ob- 
served too, that the more obliquely the ball is struck 
against the cushion, the more obliquely it rebounds on 
the opposite side, so that a billiard player can calculate 
with great accuracy in what direction it will return. 

Mrs. B. Very well. This figure (fig. 4. plate II.) 
represents a billiard table ; now if you draw a line A B 
from the point where the ball A strikes perpendicular to 
the cushion, you will find that it will divide the angle 
which the ball describes into two parts, or two angles ; 
the one will show the obliquity of the direction of the 
ball in its passage towards the cushion, the other its ob- 
liquity in its passage back from the cushion. The first 
is called the angle of incidence, the other the angle of re- 
fection, and these angles are always equal.* 

Caroline. This then is the reason why, when I throw 
a ball obliquely against the wall, it rebounds in an oppo- 
site oblique direction, forming equal angles of incidence 
and of reflection. 

Mrs. B. Certainly ; and you will find that the more 
obliquely you throw the ball, the more obliquely it will 
rebound. 

We must now conclude : but I shall have some further 
observations to make upon the laws of motion, at our next 
meeting. 



* The Angle of Incidence is that which is contained between 
the line described by the incident ray, and a line perpendicular 
to the surface on which the ray strikes, raised from the point of 
incidence. The Jingle of Reflection is that which is contained 
between the line described by the reflected ray, and a line per- 
pendicular to the reflecting surface at the point in which the in- 
cident ray strikes that surface. 

157. How does the angle of incidence compare, as to size, 

with the angle of reflection ? 158. How would you illustrate 

the angle of incidence and reflection by Fig. 4, plate II ? 159. 

What is an angle of incidence ? ICO. What is an angle of 

reflection ? 



ON COMPOUND MOTION. 51 

CONVERSATION IV. 

ON COMPOUND MOTION. 

Compound Motion, the Result of two Opposite Forces ; 
Of Circular Motion, the Result of two Forces, one of 
which confines the Body to a Fixed Point ; centre of Mo- 
tion, the Point at Rest while the other Parts of the Body 
move round it ; Centre of Magnitude, the Middle of a 
Body ; Centripetal Force, that which confines a Body 
to a fixed Central Point ; Centrifugal Force, that which 
impels a Body to fly from the Centre ; Fall of Bodies in 
a Parabola ; Centre of Gravity, the Centre of Weight, 
or point about which the Parts balance each other. 

MRS. B. 

I must now explain to you the nature of compound mo- 
tion. Let us suppose a body to be struck by two equal 
forces in opposite directions, how will it move ? 

Emily. If the directions of the forces are in exact op- 
position to each other, I suppose the body would not move 
at all. 

Mrs. B. You are perfectly right ; but if the forces, 
instead of acting on the body in opposition, strike it in 
two directions inclined to each other, at an angle of nine- 
ty degrees, if the ball A (fig. 5, plate II.) be struck by 
equal forces at X and at Y, will it not move ? 

Emily. The force X would send it towards B, and 
the force Y towards C, and since these forces are equal, I 
do not know how the body can obey one impulse rather 
than the other, and yet I think the ball would move, be- 
cause as the two forces do not act in direct opposition, 
they cannot entirely destroy the effect of each other. 

Mrs. B. Very true ; the ball will therefore follow 
the direction of neither of the forces, but will move in a 
line between them, and will reach D in the same space 
of time that the force X would have sent it to B, and the 



162. Of what does the fourth Conversation treat ? 163. 

What would be the effect if two bodies were to strike each other, 

when moving in opposite directions and with equal forces ? 

164. What would be the effect if they were to strike in directions 

inclined to each other, at an angle of ninety degrees ?■ 165 

How would you explain Fig 5, plate II. ? 



r >'2 ON COMPOUND MOTION. 

force Y would have sent it to C. Now if you draw two 
lines from D, to join B and C, you will form a square, 
and the oblique line which the body describes is called 
the diagonal of the square. 

Car f line. That is very clear, but supposing the two 
force? to be unequal, that the force X, for instance, be 
twice as great as the force Y ? 

Mrs. B. Then the force X would drive the ball twice 
as far as the force Y, consequently you must draw the 
line A B (fig. 6.,) twice as long as the line A C, the 
body will in this case move to D ; and if you draw lines 
from that point to B and C, you will find that the ball 
has moved in the diagonal of a rectangle. 

Emily. Allow me to put another case 1 Suppose the 
two forces are unequal, but do not act on the ball in the 
direction of a right angle, but in that of an acute angle, 
what will result ? 

Mrs. B. Prolong the lines in the directions of the 
two forces, and you will soon discover which way the 
ball will be impelled ; it will move from A to D, in the 
diagonal of a parallelogram, (fig. 7.) Forces acting in 
the direction of lines forming an obtuse angle, will also 
produce motion in the diagonal of a parallelogram. For 
instance, if the body set out from B, instead of A, and 
was impelled by the forces X and Y, it would move in 
the dotted diagonal B C. 

We may now proceed to circular motion : this is the 
result of two forces on a body, by one of which it is pro- 
jected forward in a right line, whilst by the other it is 
confined to a fixed point. For instance, when I whirl 
this ball, which is fastened to my hand with a string, the 
ball moves in a circular direction ; because it is acted on 
by two forces, that which I give it which represents the 
force of projection, and that of the string which confines 
it to my hand. If during its motion you were suddenly 
to cut the string, the ball would fly off in a straight line : 
being released from confinement to the fixed point, it 
would be acted on but by one force, and motion produced 
by one force, you know, is always in a right line. 

166. What is the oblique line called, which is described by two 

equal forces moving in right angular directions ? 167. "What 

does Fig. 6, of that plate illustrate ? 168. What is illustrated 

by Fig- 7, plate IT. ? 169. Of what is circular motion the re- 
sult 170. What simple instance of circular motion thus pro- 

<1 could you give ? 



ON COMPOUND MOTION. 53 

Caroline. This is a little more difficult to comprehend 
than compound motion in straight lines. 

Mrs. B. You have seen a mop trundled, and have 
observed that the threads which compose the head of 
the mop fly from the centre ; but being confined to it at 
one end, they caanot part from it ; whilst the water they 
contain, being uneonfined, is thrown off in straight lines. 

Emily. In the same way, the flyers of a windmill, 
when put in motion by the wind, would be driven 
straight forwards in a right line, were they not confined to 
a fixed point round which they are compelled to move. 

Mrs. B. Very well. And observe, that the point to 
which the motion of a small body, such as the ball with 
the string, which may be considered as revolving in one 
plane, is confined, becomes the centre of its motion. 
But when the bodies are not of a size or shape to allow 
of our considering every part of them as moving in the 
same plane, they in reality revolve round a line, which 
line is called the axis of motion. In a top, for instance, 
when spinning on its point, the axis is the line which passes 
through the middle of it, perpendicularly to the floor. 

Caroline. The axle of the flyers of the windmill is 
then the axis of its motion ; but is the centre of motion 
always in the middle of a body ? 

Mrs. B. No, not always. The middle point of a 
body is called its centre of magnitude, or position, that 
is, the centre of its mass or bulk. Bodies have also 
another centre, called the centre of gravity, which I shall 
explain to you ; but at present we must confine ourselves 
to the axis of motion. This line you must observe re- 
mains at rest, whilst all the other parts of the body move 
around it; when you spin a top the axis is stationary 
whilst every other part is in motion round it. 

Caroline. But a top generally has a motion forwards, 
besides its spinning motion ; and then no point within it 
can be at rest ? 

Mrs. B. What I say of the axis of motion relates 
only to circular motion ; that is to say, to motion round 
a line, and not to that which a body may have at the same 
time in any other direction. There is one circumstance 

171. What is meant by the axis of motion ? 172. Is the 

centre of motion always in the middle of a body ? 173. What 

is the middle point of a body called ? 174. How is the ve- 
locity of motion at different distances from the axis of motion ? 

5* 



34 ON COMPOUND MOTION. 

in circular motion, which you must carefully attend to 
which is, that the further any part of a body is from the 
axis of motion, the greater is its velocity ; as you approach 
that line, the velocity of the parts gradually diminish till 
you reach the axis of motion, which is perfectly at rest. 

Caroline. But, if every part of the same body did not 
move with the same velocity, that part which moved 
quickest, must be separated from the rest of the body, and 
leave it behind ? 

Mrs. B. You perplex yourself by confounding the 
idea of circular motion, with that of motion in a right 
line ; you must think only of the motion of a body round 
a fixed line, and you will find, that if the parts farthest 
from the centre had not the greatest velocity, those parts 
would not be able to keep up with the rest of the body, 
and would be left behind. Do not the extremities of the 
vanes of a windmill move over a much greater space 
than the parts nearest the axis of motion 1 (pi. III. fig. 
1.) The three dotted circles describe the paths in which 
three different parts of the vanes move, and though the 
circles are of different dimensions, the vanes describe each 
of them in the same space of time. 

Caroline. Certainly they do ; and I now only wonder 
that we neither of us ever made the observation before ; 
and the same effect must take place in a solid body, like 
the top in spinning ; the most bulging part of the surface 
must move with the greatest rapidity. 

Mrs. B. Trie force which confines a body to a cen- 
tre, round which it moves, is called the centripetal foi;ce ; 
and that force which impels a body to fly from the centre 
is called the centrifugal force ; in circular motion these 
two forces constantly balance each other ; otherwise the 
revolving body would either approach the centre, or re- 
cede from it, according as the one or the other prevailed 

Caroline. When I see any body moving in a circle 
I shall remember that it is acted on by two forces. 

Mrs. B. Motion, either in a circle, an ellipsis, or 
any other curve-line, must be the result of the action of 
two forces ; for you know, that the impulse of one single 
force always produces motion in a right line. 

175. What figure illustrates this ? 176. What are the 

forces called in circular motion, that balance or act in opposition 

to each other ? 177. What is meant by centripetal motion ? 

17^ What is meant by centrifugal motion ? 



ON COMPOUND MOTION. 55 

Emily. And if any cause should destroy the centripetal 
force, the centrifugal force would alone impel the body, 
and it would, I suppose, fly off in a straight line from the 
centre to which it had been confined. 

Mrs. B. It would not fly off in a right line from the 
centre ; but in a right line in the direction in which it 
was moving, at the instant of its release ; if a stone, whirl- 
ed round in a sling, gets loose at the point A (plate III. 
fig. 2.) it flies off in the direction A B ; this line is called 
a tangent, it touches the circumference of the circle, and 
forms a right angle with a line drawn from that point of 
the circumference, to the centre of the circle C. 

Emily. You say, that motion in a curve-line is owing 
to two forces acting upon a body ; but when I throw this 
ball in a horizontal direction, it describes a curve line in 
falling ; and yet it is only acted upon by the force of pro- 
jection ; there is no centripetal force to confine it, or pro- 
duce compound motion. 

Mrs. B. A ball thus thrown is acted upon by no less 
than three forces ; the force of projection, which you com- 
municated to it ; the resistance of the air through which 
it passes, which diminishes its velocity, without changing 
its direction ; and the force of gravity, which finally 
brmgs it to the ground. The power of gravity, and the 
resistance of the air, being always greater than any force 
of projection we can give a body, the latter is gradually 
overcome, and the body brought to the ground ; but the 
stronger the projectile force, the longer will these powers 
be in subduing it, and the further the body will go before 
it falls. 

Caroline. A shot fired from a cannon, for instance, 
will go much further, than a stone projected by the hand. 

Mrs. B. Bodies thus projected, you observed, describ- 
ed a curve-line in their descent ; can you account for 
that? 

Caroline. No; I do not understand, why it should 
not fall in the diagonal of a square. 

Mrs. B. You must consider that the force of projec- 
tion is strongest when the ball is first thrown ; this force, 

179. What would be the consequence, if, in circular motion, 
the centripetal should be destroyed ? 180. Which figure il- 
lustrates this ? 181. What is the line called in which a body 

would fly off, if the centripetal force were destroyed ? 182. If 

a ball is thrown horizontally, how many forces operate upon it ? 
1P3. What are they nailed? 



5G ON COMPOUND MOTION. 

as it proceeds, being weakened by the continued resist- 
ance of the air, the stone, therefore, begins by moving 
in a horizontal direction ; but as the stronger powers pre- 
vail, the direction of the ball will gradually change from 
a horizontal to a perpendicular line. Projection alone 
would drive the ball A to B, (fig. 3,) gravity would bring 
it to C ; therefore, when acted on in different directions, 
by these two forces, it moves between, gradually inclining 
more and more to the force of gravity, in proportion as 
this accumulates ; instead therefore of reaching the 
ground at D, as you supposed it would, it falls somewhere 
about E. 

Caroline. It is precisely so ; look, Emity, as I throw 
this ball directly upwards, how the resistance of the air 
and gravity conquer projection ! Now I will throw it 
upwards obliquely : see, the force of projection enables it, 
for an instant, to act in opposition to that of gravity ; but 
it is soon brought down again. 

Mrs. B. The curve-line which the ball has described, 
is called in geometry, a parabola; but when the ball is 
thrown perpendicularly upwards, it will descend perpen- 
dicularly ; because the force of projection, and that of 
gravity, are in the same line of direction. 

We have noticed the centres of magnitude, and of mo- 
tion ; but I have not yet explained to you what is meant 
by the centre of gravity ; it is that point in a body, about 
which all the parts exactly balance each other ; if, there- 
fore, that point is supported, the body will not fall. Do 
you understand this ? 

Emily. I think so ; if the parts round about this point 
have an equal tendency to fall, they will be in equilibrium, 
and as long as this point is supported, the body cannot fall, 

Mrs. B. Caroline, what would be the effect, were any 
other point of the body alone supported ? 

Caroline. The surrounding parts, no longer balancing 
each other, the body, I suppose, would fall on the side at 
which the parts are heaviest. 

Mrs. B. Infallibly ; whenever the centre of gravity 
is unsupported, the body must fall. This sometimes hap- 
pens with an overloaded wagon winding up a steep hill, 

184. How would you explain Fig. 3. plate III. ? 185. What 

is a parabola ? 18(5. Why will a stone thrown perpendicular- 
ly into the air descend perpendicularly ? 187. What is meant 

by the centre of gravity ? 188. What part of a body must be 

•upported to keep it from falling •> 



ON COMPOUND MOTION. 57 

one side of the road being more elevated than the other ; 
let us suppose it to slope as is described in this figure, 
(plate III. fig 4,) we will say, that the centre of gravity 
of this loaded wagon is at the point A. Now your 
eye will tell you that a wagon, thus situated, will over- 
set ; and the reason is, that the centre of gravity, A, is 
not supported ; for if you draw a perpendicular line from 
it to the ground at C, it does not fall under the wagon 
within the wheels, and is therefore not supported by 
them. 

Caroline. I understand that perfectly ; but what is 
the meaning of the other point B ? 

Mrs. B. Let us, in imagination, take off the upper 
part of the load ; the centre of gravity will then change 
its situation, and descend to B, as that will now be the 
point, about which the parts of the less heavily laden wa- 
gon will balance each other. Will the wagon now be 
upset ? 

Caroline. No, because a perpendicular line from that 
point falls within the wheels at D, and is supported by 
them ; and when the centre of gravity is supported, the 
body will not fall. 

Emily. Yet I should not much like to pass a wagon 
in that situation ; for, as you see, the point D is but 
just within the left wheel ; if the right wheel was merely 
raised, by passing over a stone, the point D would be 
thrown on the outside of the left wheel, and the wa- 
gon would upset. 

Caroline. A wagon, or any carriage whatever, will 
then be most firmly supported, when the centre of gra- 
vity falls exactly between the wheels ; and that is the case 
in a level road. 

Pray, whereabouts is the centre of gravity of the hu- 
man body ? 

Mrs. B. Between the hips ; and as long as we stand 
upright, this point is supported by the feet.; if you lean 
on one side, you will find that you no longer stand firm. 
A rope-dancer performs all his feats of agility, by dexte- 
rously supporting his centre of gravity ; whenever he finds 
that he is in 4 an ger of losing his balance, he shifts the 
heavy pole, which he holds in his hands, in order to throw 

189. What explanation would you give of Fig. 4, plate III. ? 

190. Why do persons in ascending a hill incline forward, and 

in descending it incline backward ? 191. How is it that rope- 
dancers are able to perform their feats of agility without falling ? 



58 ON COMPOUND MOTION. 

the weight towards the side that is deficient ; and thus by 
changing the situation. of the centre of gravity, he restores 
his equilibrium. 

Caroline. When a stick is poised on the tip of the 
finger, is it not by supporting its centre of gravity ? 

Mrs* B. Yes ; and it is because the centre of gravity 
is not supported, that spherical bodies roll down a slope. 
A sphere being perfectly round, can touch the slope but 
by a single point, and that point cannot be perpendicularly 
under the centre of gravity, and therefore cannot be sup- 
ported, as you will perceive by examining this figure, (fig. 
5. plate III.) 

Emily. So it appears ; yet I have seen a cylinder of 
wood roll up a slope ; how is that contrived ? 

Mrs. B. It is done by plugging one side of the cylin- 
der with lead, as at B. (fig. 5. plate III.) the body being 
no longer of a uniform density, the centre of gravity is 
removed from the middle of the body to some point in the 
lead, as that substance is much heavier than wood ; now 
you may observe that in order that the cylinder may roll 
down the plane, as it is here situated, the centre of gra- 
vity must rise, which is impossible ; the centre of gravity 
must always descend in moving, and will descend by the 
nearest and readiest means, which will be by forcing the 
cylinder up the slope, until the centre of gravity is sup- 
ported, and then it stops. 

Caroline. The centre of gravity, therefore, is not al- 
ways in the middle of a body. 

Mrs. B. No, that point we have called the centre of 
magnitude ; when the body is of a uniform density the 
centre of gravity is in the same point ; but when one part 
of the body is composed of heavier materials than another 
part, the centre of gravity being the centre of the weight 
of the body can no longer correspond with the centre of 
magnitude. Thus you see the centre of gravity of this 
cylinder, plugged with lead, cannot be in the same spot as 
the centre of magnitude. 

Emily. Bodies, therefore, consisting but of one kind 

192. Why do spherical bodies roll down a slope or inclined 

plane ? 193. By which figure is this illustrated ? 194. 

How can a cylinder of wood be made to roll up a slope ? 

196. Is the centre of gravity always the centre of magnitude ? 

190. When is the centre of gravity in the same point with 

the centre of magnitude ? 197. When will they not bo in 

the same point ? 



ON COMPOUND MOTION. 59 

of substance, as wood, stone, or lead, and whose densities 
are consequently uniform, must stand more firmly, and 
be more difficult to overset, than bodies composed of a 
variety of substances, of different densities, which may 
throw the centre of gravity on one side. 

Mrs. B. Yes; but there is another circumstance 
which more materially affects the firmness of their position, 
and that is their form. Bodies that have a narrow base 
are easily upset, for if they are the least inclined, their 
centre is no longer supported, as you may perceive in 
fig. 6. 

Caroline. I have often observed with what difficulty 
a person carries a single pail of water ; it is owing, 1 
suppose, to the centre of gravity being thrown on one side, 
and the opposite arm is stretched out to endeavour to bring 
it back to its original situation ; but a pail hanging on 
each arm is carried without difficulty, because they ba- 
lance each other, and the centre of gravity remains sup- 
ported by the feet. 

Mrs. B. Very well ; I have but one more remark to 
make on the centre of gravity, which is, that when two 
bodies are fastened together, by a line, string, chain, or any 
power whatever, they are to be considered as forming but 
one body ; if the two bodies be of equal weight, the centre 
of gravity will be in the middle of the line which unites 
them, (fig. 7,) but if one be heavier than the other, the 
centre of gravity will be proportionally nearer the heavy 
body than the light one. (fig. 8.) If you were to carry a rod 
or pole with an equal weight fastened at each end of it, 
you would hold it in the middle of the rod, in order that 
the weights should balance each other ; whilst if it had 
unequal weights at each end, you would hold it nearest 
the greater weight, to make them balance each other. 

Emily. And in both cases we should support the cen- 
tre of gravity ; and if one weight be very considerably 
larger than the other, the centre of gravity will be thrown 
out of the rod into the heaviest weight, (fig. 9.) 

Mrs. B. Undoubtedly. 

198. What bodies stand most firmly, and what ones are most 

easily upset ? 199. What is the object of Fig. 6, plate III. ? 

200. Why can a person carry two pails of water, one in each 

hand, easier than a single pail ? 201. If two bodies are connect- 
ed together, how are they to be considered as to their centre of gra- 
vity ? 202. If they are of equal weight, where will the centre of 

gravity be ? 203. If they are of unequal weight, where will it 

be ? 201. What is the object of Fig. 7, 8, and 9, of plate III. ? 



60 ON THE MECHANICAL POWERS. 

CONVERSATION V. 

ON THE MECHANICAL POWERS. 

Of the Power of Machines ; Of the Lever in General; Of 
the Lever of the First Kind, having the Fulcrum be- 
ticeen the Power and the Weight ; Of the Lever of the 
Second Kind, having the Weight between the Powe\ 
and the Fulcrum ; Of the Lever of the Third Kim 
having the Power bcttceen the Fulcrum and the Weight 



MRS. B. 



We may now proceed to examine the mechanical pow- 
ers ; they are six in number, one or more of which enter: 
into the composition of every machine. The lever, the 
pulley, the wheel, and axle, the inclined plane, the wedgc r 
and the screw. 

In order to understand the power of a machine, there 
are four things to be considered. 1st. The power that 
acts : this consists in the effort of men or horses, of 
weights, springs, steam, &c. 

2dly. The resistance which is to be overcome by the 
power ; this is generally a weight to be moved. The 
power must always be superiour to the resistance, other- 
wise the machine could not be put in motion. 

Caroline. If, for instance, the resistance of a carriage 
was greater than the strength of the horses employed to 
draw it, they would not be able to make it move. 

Mrs. B. 3dly. We are to consider the centre of mo- 
tion, or as it is, termed in mechanicks, the fulcrum; this, 
you may recollect, is the point about which all the parts 
of the body move ; and lastly, the respective velocities 
of the power, and of the resistance. 

Emily. That must depend upon their respective dis- 
tances from the axis of motion ; as we observed in the 
motion of the vanes of the windmill. 

Mrs. B. We shall now examine the power of the 
lever. The lever is an inflexible rod or beam of any kind, 
that is to say, one which will not bend in any direction, 

205. How many of the mechanical powers are there ?< 

906. What are the names of them ? 207. In order to un- 
derstand the power of a machine, how many things are to be 

dered ? WS. What is the first r — the second ? — the third 3 

209. What is the lever ? 



i 



i 



ON THE MECHANICAL POWERS. 61 

For instance, the steel rod to which these scales are sus- 
pended is a lever, and the point in which it is supported 
the fulcmm, or centre of motion ; now, can you tell me 
why the two scales are in equilibrium ? 

Caroline, Being both empty, and of the same weight, 
they balance each other. 

Emily. Or, more correctly speaking, because the 
centre of gravity common to both is supported. 

Mrs. B. Very well ; and which is the centre of gra- 
vity of this pair of scales? (fig. 1. plate IV.) 

Emily. You have told us that when two bodies of 
equal weight were fastened together, the centre of gravity 
was in the middle of the line that connected them ; the 
centre of gravity of the scales must therefore be in the ful- 
crum F of the lever which unites the two scales ; and cor- 
responds with the centre of motion. 

Caroline. But if the scales contained different weights, 
the centre of gravity would no longer be in the fulcrum of 
the lever, but removed towards that scale which contained 
the heaviest weight ; and since that point would no longer 
be supported, the heavy scale would descend and out- 
weigh the other. 

Mrs. B. True ; but tell me, can you imagine any 
mode by which bodies of different weights can be made to 
balance each other, either in a pair of scales, or simply 
suspended to the extremities of the lever ? for the scales 
are not an essential part of the machine, they have no me- 
chanical power, and are used merely for the convenience 
of containing the substance to be weighed. 

Caroline. What ! make a light body balance a heavy 
one ? I cannot conceive that possible. 

Mrs. B. The fulcrum of this pair of scales (fig. 2.) is 
moveable, you see ; I can take it off the prop, and fasten 
it on again in another part ; this part ia now become the 
fulcrum, but it is no longer in the centre of the lever. 

Caroline. And the scales are no longer true ; for that 
which hangs on the longest side of the lever descends. 

Mrs. B. The two parts of the lever divided by the ful- 
crum a-*e called its arms, you should therefore say the 
longest arm, not the longest side of the lever. These 

210. Why are the scales as seen in Fig. 1, plate IV. in equi- 
librium ? 211. What is the centre of gravity to two scales in 

equilibrium as seen in that figure ? 212. What are xhe arms 

of a lever ? 



C2 ON THE MECHANICAL POWERS. 

arms are likewise frequently distinguished by the appella 
tions of the acting and the resisting part of the lever. 

Your observation is true that the balance is now de- 
stroyed ; but it will answer the purpose of enabling you 
to comprehend the power or a lever when the fulcrum is 
not in the centre. 

Emily. This would be an excellent contrivance for 
those who cheat in the weight of their goods ; by making 
the fulcrum a little on one side, and placing the goods in 
the scale which is suspended to the longest arm of the 
lever, they would appear to weigh more than they do in 
reality. 

Mrs. B. You do not consider how easily the fraud 
would be detected ; for on the scales being emptied, they 
would not hang in equilibrium. 

Emily. True ; I did not think of that circumstance. 
But I do not understand why the longest arm of the lever 
should not be in equilibrium with the other. 

Caroline. It is because it is heavier than the shortest 
arm ; the centre of gravity, therefore, is no longer sup- 
ported. 

Mrs. B. You are right ; the fulcrum is no longer in 
the centre of gravity ; but if we can contrive to make the 
fulcrum in its present situation become the centre of gra- 
vity, the scales will again balance each other ; for you 
recollect that the centre of gravity is that point about 
which every part of the body is in equilibrium. 

Emily. It has just occurred to me how this may be 
accomplished ; put a great weight into the scale suspended 
to the shortest arm of the lever, and a smaller one into 
that suspended to the longest arm. Yes, I have disco- 
vered it — look, Mrs. B., the scale on the shortest arm will 
carry 21bs., and that on the longest arm only one, to re- 
store the balance, (fig. 3.) 

Mrs. B. You see, therefore, that it is not so imprac- 
ticable as you imagined to make a heavy body balance a 
light one ; and this is in fact the means by which you 
thought an imposition in the weight of goods might be 
effected, as a weight of ten or twelve ounces mi^ht thus 
be made to balance a pound of goods. Let us now take 

213. What is the reason that the arms of the lever, as seen 

Fig. 2, plate IV. are not supported ? 214. In what way can 

they be made to support each other r 215. What is illustrated 

by Fig. 3, plate IV. ? 



ON THE MECHANICAL POWERS. 63 

off the scales that we may consider the lever simply ; and 
in this state you see that the fulcrum is no longer the cen- 
tre of gravity ; but it is, and must ever be, the centre of 
motion, as it is the only point which remains at rest, 
while the other parts move about it. 

Caroline. It now resembles the two opposite vanes of 
a windmill, and the fulcrum the point round which they 
move. 

Mrs. B. In describing the motion of those vanes, you 
may recollect ouc observing that the further a body is 
from the axis of motion, the greater is its velocity. 

Caroline. That I remember and understood perfectly. 

Mrs. B. You comprehend then, that the extremity 
of the longest arm of a lever must move with greater 
velocity than that of the shortest arm ? 

Emily. No doubt, because it is furthest from the cen- 
tre of motion. And pray, Mrs. B., when my brothers 
play at see-saw, is not the plank on which they ride a 
kind of lever 1 

Mrs. B. Certainly ; the log of wood which supports 
it from the ground is the fulcrum, and those who ride 
represent the power and the resistance at each end of 
the lever. And have you not observed that when those 
who ride are of equal weight, the plank must be sup- 
ported in the middle to make the two arms equal ; whilst 
if the persons differ in weight, the plank must be drawn 
a little further over the prop, to make the arms unequal, 
and the lightest person who represents the resistance, 
must be placed at the extremity of the longest arm. 

Caroline. That is always the case when I ride on a 
plank with my youngest brother ; I have observed also 
that the lightest person has the best ride, as he moves 
both further and quicker ; and I now understand that it 
is because he is more distant from the centre of motion. 

Mrs. B. The greater the velocity with which your little 
brother moves, renders his momentum equal to yours. 

Caroline. Yes ; I have the most gravity, he the great- 
est velocity ; so that upon the whole our momentums are 
equal. But you said, Mrs. B., that the power should be 
greater than the resistance to put the machine in motion ; 
how then can the plank move if the momentums of the 
persons who ride are equal 1 

216. What is the velocity of the extremity of the longest arm 
of a lever compared with that of the shortest arm ? 



G4 ON THE MECHANICAL POWERS. 

Mrs. B. Because each person at his descent touches 
the ground with his feet ; the re-action of which gives him 
an impulse which increases his velocity ; this spring is 
requisite to destroy the equilibrium of the power and the 
resistance, otherwise the plank would not move. Did 
you ever observe that a lever describes the arc of a circle 
in its motion 1 

Emily. No ; it appears to me to rise and descend 
perpendicularly ; at least I always thought so. 

Mrs. B. I believe I must make a sketch of you and 
your brother riding on a plank, in order to convince you 
of your error, (fig. 4, pi. IV.) You may now observe 
that a lever can move only round the fulcrum, since that 
is the centre of motion ; it would be impossible for you 
to rise perpendicularly to the point A, or for your brother 
to descend in a straight line to the point B ; you must 
in rising and he in descending describe arcs of your 
respective circles. This drawing shows you also how 
much superiour his velocity must be to yours ; for if you 
could swing quite round, you would each complete your 
respective circles in the same time. 

Caroline. My brother's circle being much the largest, 
he must undoubtedly move the quickest. 

Mrs. B. Now "tell me, do you think that your brother 
could raise you as easily without the aid of a lever ? 

Caroline. Oh no, he could not lift me off the ground. 

Mrs. B. Then I think you require no further proof 
of the power of a lever, since you see what it enables your 
brother to perform. 

Caroline. I now understand what you meant by say- 
ing, that in mechanicks, motion was opposed to matter, 
for it is my brother's velocity which overcomes my weight. 

Mrs. B. You may easily imagine, what enormous 
weights may be raised by levers of this description, for 
the longer the acting part of the lever in comparison to 
the resisting part, the greater is the effect produced by 
it ; because the greater is the velocity of the power com- 
pared to that of the weight. 

There are three different kinds of levers ; in the first 
the fulcrum is between the power and the weight. 

217. What does a lever in its motion describe ? 218. What 

is the design of Fig. 4, plate IV. ? 219. To what is the great- 
ness of effect produced by the lever proportional ? 220. How 

many kinds ot levers are there ? 



ON THE MECHANICAL POWERS. 65 

Caroline. This kind then comprehends the several 
levers you have described. 

Mrs. B. Yes, when in levers of the first kind, the ful- 
crum is equally between the power and the weight, as in 
the balance the power must be greater than the weight, 
in order to move it ; for nothing can in this case be 
gained by velocity ; the two arms of the lever being equal, 
the velocity of their extremities must be so likewise. The 
balance is therefore of no assistance as a mechanical 
power, but it is extremely useful to estimate the respective 
weights of bodies. 

But when (fig. 5.) the fulcrum F of a lever is not equally 
distant from the power and the weight, and that the power 
P acts at the extremity of the longest arm, it may be less 
than the weight W, its deficiency being compensated by 
its superiour velocity ; as we observed in the seesaw. 

Emily. Then when we want to lift a great weight, 
we must fasten it to the shortest arm of a lever, and apply 
our strength to the longest arm ? 

Mrs. B. If the case will admit of your putting the 
end of the lever under the weight, no fastening will be re- 
quired ; as you will perceive by stirring the fire. 

Emily. Oh yes ! the poker is a lever of the first kind, 
the point where it rests against the bars of the grate, whilst 
I am stirring the fire, is the fulcrum ; the short arm or 
resisting part of the lever is employed in lifting the 
weight, which is the coals, and my hand is the power ap- 
plied to the longest arm, or acting part of the lever. 

Mrs. B. Let me hear, Caroline, whether you can 
equally well explain this instrument, which is composed 
of two levers, united in one common fulcrum. 

Caroline. A pair of scissors ! 

Mrs. B. You are surprised, but if you examine their 
construction, you will discover that it is the power of the 
lever that assists us in cutting with scissors. 

Caroline. Yes ; I now perceive that the point at 
which the two levers are screwed together, is the fulcrum ; 
the handles, to which the power of the fingers is applied, 



221. Where is the fulcrum in the first kind ?— — 222. How 

are we to use levers of the first kind in raising large weights ? 

223. What power of mechanicks do the common scissors involve ? 

224 How may the scissors be explained as formed by the 

lever ? 6 * 



00 ON THE MECHANICAL POWERS. 

are the extremities of the acting part of the levers, and 
the cutting part of the scissors, are the resisting parts of 
the levers : therefore, the longer the handles and the 
shorter the points of the scissors, the more easily you cut 
with them. 

Emily. That I have often observed, for when I cut 
pasteboard or any hard substance, I always make use of 
that part of the scissors nearest the screw or rivet, and 

1 now understand why it increases the power of cutting ; 
but I confess I never should have discovered scissors to 
have been double^ levers ; and pray are not snuffers 
levers of a similar description ? 

Mrs. B. Yes, and most kinds of pincers ; the great 
power of which consists in the resisting part of the lever 
being very short in comparison of the acting part. 

Caroline. And of what nature are the two other kinds 
of levers ? 

Mrs. B. In levers of the second kind, the weight, 
instead of being at one end, is situated between the power 
and the fulcrum, (fig. 0.) 

Caroline. The weight and the fulcrum have here 
changed places ; and what advantage is gained by this 
kind of lever 1 

Mrs. B. In moving it, the velocity of the power must 
necessarily be greater than that of the weight, as it is more 
distant from the centre of the motion. 

Have you ever seen your brother move a snow-ball by 
means of a strong stick, when it became too heavy for 
him to move without assistance 1 

Caroline. Oh yes ; and this was a lever of the second 
order (fig. 7.) ; the end of the stick, which he thrusts 
under the ball, and which rests on the ground, becomes 
the fulcrum ; the ball is the weight to be moved, and the 
power his hands applied to the other end of the lever. 
In this instance there is an immense difference in the 
length of the arms of the lever ; for the weight is almost 
close to the fulcrum. 

Mrs. B. And the advantage gained is proportional 
to this difference. Fishermen's boats are by levers of 
this description raised from the ground to be launched 

225. How is the second kind of lever designated > 226 

Which figures illustrate the use of levers of the second kind? 
227. To what is the advantage gained in the use of the se- 
cond kind oflevor proportional ? 



ON THE MECHANICAL POWERS. 67 

into the sea, by means of slippery pieces of board which 
are thrust under the keel. The most common example 
that we have of levers of the second kind is in the doors 
of our apartments. 

Emily. The hinges represent the fulcrum, our hands 
the power applied to the other end of the lever ; but 
where is the weight to be moved ? 

Mrs. B. The door is the weight, and it consequently 
occupies the whole of the space between the power and 
the fulcrum. Nut-crackers are double levers of this 
kind ; the hinge is the fulcrum, the nut the resistance, 
and the hands the power. 

In levers of the third kind, (fig. 8.), the fulcrum is again 
at one of the extremities, the weight or resistance at the 
other, and it is now the power which is applied between 
the fulcrum and the resistance. 

Emily. The fulcrum, the weight, and the power, then, 
each in their turn, occupy some part of the middle of the 
lever between its extremities. But in this third kind of 
lever, the weight being further from the centre of motion 
than the power, the difficulty of raising it seems increased 
rather than diminished. 

Mrs. B. That is very true ; a lever of this kind is 
therefore never used, unless absolutely necessary, as is 
the case in lifting up a ladder perpendicularly in order to 
place it against the wall ; the man who raises it cannot 
place his hands on the upper part of the ladder, the power, 
therefore, is necessarily placed much nearer the fulcrum 
than the weight. 

Caroline. Yes, the hands are the power, the ground 
the fulcrum, and the upper part of the ladder the weight. 

Mrs. B. Nature employs this kind of lever in the 
structure of the human frame. In lifting a weight with 
the hand, the lower part of the arm becomes a lever of 
the third kind : the elbow is the fulcrum, the muscles of 
the fleshy part of the arm the power ; and as these are 
nearer to the elbow than the hand, it is necessary that 
their power should exceed the weight to be raised. 

228. What are the most common examples of levers of the se- 
cond kind ? 229. How would you explain the opening of a 

common door, as involving the principle of the second kind of le- 
vers ?— 230. What is the third kind of levers ? 231. What 

is an instance of its use ? 232. How does the raising jf a 

weight by the hand represent this kind of levers t 



68 ON THE MECHANICAL POWERS. 

Emily. Is it not surprising tlKit nature should have 
furnished us with such disadvantageous levers. 

Mrs. B. The disadvantage, in respect to power, is 
more than counterbalanced by the convenience resulting 
from this structure of the arm : and it is no doubt that 
which is best adapted to enable it to perform its various 
functions. 

We have dwelt so long on the lever, that we must re- 
serve the examination of the other mechanical powers to 
our next interview. 



CONVERSATION V. 

CONTINUED. 

ON THE MECHANICAL POWERS. 

Of the Pulley ; Of the Wheel and Axle ; Of the Inclined 
Plane ; Of the Wedge ; Of the Screw. 

MRS. B. 

The pulley is the second mechanical power we are to 
examine. You both, I suppose, have seen a pulley ? 

Caroline. Yes, frequently : it is a circular and flat 
piece of wood or metal, with a string which runs in a 
groove round it ; by means of which, a weight may be 
pulled up ; thus pulleys are used for drawing up curtains. 

Mrs. B. Yes; but in that instance the pulleys are 
fixed, and do not increase the power to raise the weights, 
as you will perceive by this figure, (pi. V. fig. 1.) Observe 
that the fixed pulley is on the same principle as the lever 
of a pair of scales, in which the fulcrum F being in the 
centre of gravity, the power P and the weight W, are 
equally distant from it, and no advantage is gained. 

Emily. Certainly ; if P represents the power employ- 
ed to raise the weight W, the power must be greater than 
the weight in order to move it. But of what use then are 
pulleys in mechanicks ? 

233. What is the second mechanical power ? 234. What 

is a pulley ? 235. How does Fig. 1. plate V. illustrate the fixed 

pulley ? 236. How must the power compare with the weight 

in order to move it, by the use of the fixed pulley ? 



ON THE MECHANICAL POWERS. G9 

Mrs. B. The next figure represents a pulley which is 
not fixed, (fig. 2.) and thus situated you will perceive that 
it affords us mechanical assistance. In order to raise the 
weight (W) one inch, P, the power, must draw the strings 
B and C one inch each : the whole string is therefore 
shortened two inches, while the weight is raised only one. 

Emily. That I understand : if P drew the string but 
one inch, the weight would be raised only half an inch, 
because it would shorten the strings B and C half an inch 
each, and consequently the pulley, with the weight at- 
tached to it, can be raised only half an inch. 

Caroline. I am ashamed of my stupidity ; but I con- 
fess that I do not understand this ; it appears to me that 
the weight would be raised as much as the string is short- 
ened by the power. 

Mrs. B. I will endeavour to explain it more clearly. 
I fasten this string to a chair, and draw it towards me ; I 
have now shortened the string, by the act of drawing it, 
one yard. 

Caroline. And the chair, as I supposed, has advanced 
one yard. 

Mrs. B. This exemplifies the nature of a single fixed 
pulley only. Now unfasten the string, and replace the 
chair where it stood before. In order to represent the 
moveable pulley, we must draw the chair forwards by put- 
ting the string round it ; one end of the string may be fas- 
tened to the leg of the table, and I shall draw the chair by 
the other end of the string. I have again shortened 
the string one yard ; how much has the chair advanced 1 

Caroline. I now understand it ; the chair represents 
the weight to which the moveable pulley is attached ; 
and it is very clear that the weight can be drawn only 
half the length you draw the string. I believe the cir- 
cumstance that perplexed me was, that I did not observe 
the difference that results from the weight being attached 
to the pulley, instead of being fastened to the string, as 
is the case in the fixed pulley. 

Emily. But I do not yet understand the advantage of 
pulleys ; they seem to me to increase rather than diminish 
the difficulty of raising weights, since you must draw the 
string double the length that you raise the weight ; whilst 

237. What kind of pulley does Fig. 2, plate V, represent; and 
how would you explain it ? 



70 ON THE MECHANICAL POWERS. 

with a single pulley, or without any pulley, the weight is 
raised as much as the string is shortened. 

Mrs. B. The advantage of a moveable pulley consists 
in dividing the difficulty ; we must draw, it is true, twice 
the length of the string, but then only half the strength 
is required that would be necessary to raise the weight 
without the assistance of a moveable pulley. 

Emily. So that the difficulty is overcome in the same 
manner as it would be, by dividing the weight into two 
equal parts, and raising them successively. 

Mrs. B. Exactly. You must observe, that with a 
moveable pulley the velocity of the power is double that of 
the weight, since the power P (fig. 2.) moves two inches, 
whilst the weight W moves one inch; therefore the 
power need not be more than half the weight to make 
their momentums equal. 

Caroline. Pulleys act then on the same principle as 
the lever, the deficiency of strength of the power being 
compensated by its superiour velocity. 

Mrs. B. You will find that all mechanical power is 
founded on the same principle. 

Emily. But may it not be objected to pulleys, that a 
longer time is required to raise a weight by their aid than 
without it ; for what you gain in power you lose in time ? 

Mrs. B. That, my dear, is the fundamental law in 
mechanicks : it is the case with the lever as well as the 
pulley ; and you will find it to be so with all the other 
mechanical powers. 

Caroline. I do not see any advantage in the mecha- 
nical powers then, if what we gain by them one way is lost 
another. 

Mrs. B. Since we are not able to increase our natu- 
ral strength, is not that science of wonderful utility, by 
means of which we may reduce the resistance or weight 
of any body to the level of our strength ? This the 
mechanical powers enable us to accomplish, by dividing 
the resistance of a body into parts which we can succes- 

238. In what does the advantage of a moveable pulley consist ? 

i 239. How do the weight and power of a moveable pulley com 

pare, that their momenta be equal ? 240. On what principle 

are all mechanical powers founded? 241. Is there any loss 

of time in the use of the moveable pulley ? 242. And to what 

is this loss of time proportional ? 243. What then is the ad- 
vantage of this pulley, or of any of the mechanical powers, if there 
is as much loss in time as gain in power ? 



ON THE MECHANICAL POWERS. 71 

sively overcome. It is true, as you observe, that it 
requires a sacrifice of time to attain this end, but you 
must be sensible how very advantageously it is exchanged 
for power ; the utmost exertion we can make adds but 
little to our natural strength, whilst we have a much 
more unlimited command of time. You can now under- 
stand, that the greater the number of pulleys connected 
by a string, the more easily the weight is raised, as the 
difficulty is divided among the number of strings, or rather 
of parts into which the string is divided by the pulleys. 
Several pulleys thus connected, form what is called a sys- 
tem, or tackle of pulleys, (fig. 3.) You may have seen 
them suspended from cranes to raise goods into ware- 
houses, and in ships to draw up the sails. 

Emily. But since a fixed pulley affords us no mecha- 
nical aid, why is it ever used ? 

Mrs. B. Though it does not increase our power, it 
is frequently useful for altering its direction. A single 
pulley enables us to draw up a curtain, by drawing down 
the string connected with it ; and we should be much at 
a loss to accomplish this simple operation without its as- 
sistance. 

Caroline. There would certainly be some difficulty 
in ascending to the head of the curtain, in order to draw 
it up. Indeed, I now recollect having seen workmen 
raise small weights by this means, which seemed to an- 
swer a very useful purpose. 

Mrs. B. In shipping, both the advantages of an in- 
crease of power and a change of direction, by means of 
pulleys, are united : for the sails are raised up the masts 
by the sailors on deck, from the change of direction which 
the pulley effects, and the labour is facilitated by the me- 
chanical power of a combination of pulleys. 

Emily. But the pulleys on ship-board do not appear 
to me to be united in the manner you have shown us. 

Mrs. B. They are, I believe, generally connected as 
described in figure 4, both for nautical, and a variety of 
other purposes ; but in whatever manner pulleys are con- 
nected by a single string, the mechanical power is the 
same. 



244. What is a system or tackle of pulleys, and which figure 

exhibits it ? 245. If there is no mechanical aid from the fixed 

pulley, why is it used ? 



78 ON THE MECHANICAL POWERS. 

The third mechanical power is the wheel and axle* 
Let us suppose (plate V. fig. 5.) the weight W to be a 
bucket of water in a well, which we raise by winding the 
rope, to which it is attached, round the axle ; if this be 
done without a wheel to turn the axle, no mechanical as- 
sistance is received. 

The axle without a wheel is as impotent as a single 
fixed pulley, or a lever, whose fulcrum is in the centre ; 
but add the wheel to the axle, and you will immediately 
find the bucket is raised with much less difficulty. 

The velocity of the circumference of the wheel is as 
much greater than that of the axle, as it is further from the 
centre of motion : for the wheel describes a great circle 
in the same space of time that the axle describes a small 
one, therefore the power is increased in the same pro- 
portion as the circumference of the wheel is greater than 
that of the axle. If the velocity of the wheel is twelve 
times greater than that of the axle, a power nearly twelve 
times less than the weight of the bucket would be able to 
raise it. 

Emihj. The axle acts the part of the shorter arm of 
the lever, the wheel that of the longer arm. 

Caroline. In raising water there is commonly, I be- 
lieve, instead of a wheel attached to the axle, only a 
crooked handle, which answers the purpose of winding 
the rope round the axle, and thus raising the bucket. 

Mrs. B. In this manner (fig. 6.); now if you observe 
the dotted circle which the handle describes in winding 
up the rope, you will perceive that the branch of the han- 
dle A, which is united to the axle, represents the spoke 
of a wheel, and answers the purpose of an entire wheel ; 
the other branch B affords no mechanical aid, merely 
serving as a handle to turn the wheel. 

Wheels are a very essential part to most machines : 
they are employed in various ways ; but, when fixed to 
the axle, their mechanical power is always the same ; 
that is, as the circumference of the wheel exceeds that of the 
axle, so much will the energy of its power be increased. 

Caroline. Then the larger the wheel the greater must 
be its effect. 

24(5. What is the third mechanical power ? 247. What 

docs Fig. 5, plate V. illustrate ? 248. In what proportion is the 

power of the wheel increased ? 249. How may a wheel be 

< ompared to the lever ? 250. How does Fig. 6, plate V. repre 

a wheel ? 



ON THE MECHANICAL POWERS. 73 

Mrs. B. Certainly. If you have ever seen any con- 
siderable mills or manufactures, you must have admired 
the immense wheel, the revolution of which puts the 
whole of the machinery into motion ; and though so great 
an effect is produced by it, a horse or two has sufficient 
power to turn it ; sometimes a stream of water is used for 
that purpose, but of late years, a steam-engine has been 
found both the most powerful and the. most convenient 
mode of turning the wheel. 

Caroline. Do not the vanes of a windmill represent a 
wheel, Mrs. B. 1 

Mrs. B. Yes ; and in this instance we have the ad- 
vantage of a gratuitous force, the wind, to turn the 
wheel. One of the great benefits resulting from the use 
of machinery is, that it gives us a sort of empire over the 
powers of nature, and enables us to make them perform 
the labour which would otherwise fall to the lot of man. 
When a current of wind, a stream of water, or the ex- 
pansive force of steam, performs our task, we have only 
to superintend and regulate their operations. 

The fourth mechanical power is the inclined plane ; 
this is nothing more than a slope, or declivity, frequently 
used to facilitate the drawing up of weights. It is not 
difficult to understand, that a weight may much more 
easily be drawn up a slope than it can be raised the same 
height perpendicularly. But in this, as well as the other 
mechanical powers, the facility is purchased by a loss of 
time, (fig. 7.) ; for the weight, instead of moving directly 
from A to C, must move from B to C, and as the length 
of the plane is to its height, so much is the resistance of 
the weight diminished. 

Emily. Yes ; for the resistance, instead of being con- 
fined to the short line A C, is spread over the long line 
BC. 

Mrs. B. The wedge, which is the next mechanical 
power, is composed of two inclined planes, (fig. 8.) : you 

251. On what mechanical force is the wind-mill operated ? 

252. What is found to be the most powerful and convenient 

mode of turning the wheel ? 253. What is one of the great 

benefits resulting from the use of machinery ? 254. What is 

the fourth mechanical power? 255. What is an inclined 

plane I 256. How would you explain Fig. 7, plate V. ? 

257. How much is the resistance of the weight diminished by the 
use of the inclined plane ? -258. Of what is the wedge com- 
posed : 

7 



74 ON THE MECHANICAL POWERS. 

may have seen wood-cutters use it to cleave wood. The 
resistance consists in the cohesive attraction of the wood, 
or any other body which the wedge is employed to sepa- 
rate ; and the advantage gained by this power is in the 
proportion of half its width to its length ; for while the 
wedge forces asunder the coherent particles of the wood 
to A and B, it penetrates downwards as far as C. 

Emily. The wedge, then, is rather a compound than 
a distinct mechanical power, since it is composed of two 
inclined planes. 

Mrs. B. It is so. All cutting instruments are con- 
structed upon the principle of the inclined plane, or the 
wedge : those that have but one edge sloped, like the 
chisel, may be referred to the inclined plane ; whilst the 
axe, the hatchet, and the knife (when used to split asun- 
der) are used as wedges. 

Caroline. But a knife cuts best when it is drawn across 
the substance it is to divide. We use it thus in cutting 
meat, we do not chop it to pieces. 

Mrs. B. The reason of this is, that the edge of a 
knife is really a very fine saw, and therefore acts best 
when used like that instrument. 

The screw, which is the last mechanical power, is more 
complicated than the others. You will see by this figure, 
(fig. 9.) that it is composed of two parts, the screw and the 
nut. The screw S is a cylinder, with a spiral protuberance 
coiled round it, called the thread ; the nut N is perforated 
to contain the screw, and the inside of the nut has a 
spiral groove made to fit the spiral thread of the screw. 

Caroline. It is just like this little box, the lid of which 
screws on the box as you have described ; but what is 
this handle which projects from the nut 1 

Mrs. B. It is a lever, which is attached to the nut, 
without which the screw is never used as a mechanical 
power : the nut with a lever L attached to it is commonly 
called a winch. 

The power of tYie screw, complicated as if appears, is 
referrible to one of the most simple of the mechanical 
powers ; which of them do you think it is ? 

259 In what does the resistance of the wedge consist ? 

260. On what mechanical principles are cutting instruments de- 
signed ? 261. Which is the last mechanical power ? 262. 

Of what is the screw composed ? 263. What is the construc- 
tion of the screw and nut ? 264. How would you explain Fig. 

9, plate V. ? 



ON THE MECHANICAL POWERS. 75 

Caroline. In appearance, it most resembles the wheel 
and axle. 

3Irs. B. The lever, it is true, has the effect of a 
wheel, as it is the means by which you wind the nut 
round; but the lever is not considered as composing a 
part of the screw, though it is true, that it is necessarily 
attached to it. But observe, that the lever, considered as 
a wheel, is not fastened to the axle or screw, but moves 
round it, and in so doing, the nut either rises or descends, 
according to the way in which you turn it. 

Emily. The spiral thread of the screw resembles, I 
think, an inclined plane : it is a sort of slope, by means 
of which the nut ascends more easily than it would do 
if raised perpendicularly ; and it serves to support it 
when at rest. 

Mrs. B. Very well : if you cut a slip of paper in the 
form of an inclined plane, and wind it round your pencil, 
which will represent the cylinder, you will find that it 
makes a spiral line, corresponding to the spiral protu- 
berance of the screw, (fig. 10.) 

Emily. Very true ; the nut then ascends an inclined 
plane, but ascends it in a spiral, instead of a straight line ; 
the closer the thread of the screw, the more easy the as- 
cent ; it is like having shallow, instead of steep steps to 
ascend. 

Mrs. B. Yes, excepting that the nut takes no steps, 
it gradually winds up or down ; then observe, that the clo- 
ser the threads of the screw, the greater the number of 
revolutions the winch must make ; so that we return to the 
old principle, — what is saved in power is lost in time. 

Emily. Cannot the power of a screw be increased 
also, by lengthening the lever attached to the nut ? 

Mrs. B. Certainly. The screw, with the addition of 
the lever, forms a very powerful machine, employed either 
for compression, or to raise heavy weights. It is used by 
book-binders, to press the leaves of books together ; it is 
used also in cider and wine presses, in coining, and for a 
variety of other purposes. 

All machines are composed of one or more of these six 
mechanical powers we have examined : I have but one 

265. To which of the other mechanical powers is the screw 
referrible ? 266. How can the power of the screw be in- 
creased ? 



76 ON THE MECHANICAL POWERS. 

more remark to make to you relative to them, which is, 
that friction in a considerable degree diminishes their 
force, allowance must therefore always be made for it in 
the construction of machinery. 

Caroline. By friction, do you mean one part of the 
machine rubbing against another part contiguous to it 1 

Mrs. B. Yes ; friction is the resistance which bodies 
meet with in rubbing against each other ; there is no such 
thing as perfect smoothness or evenness in nature : polished 
metals, though they wear that appearance more than any 
other bodies, are far from really possessing it ; and their 
inequalities may frequently be perceived through a good 
magnifying glass. When, therefore, the surfaces of the 
two bodies come into contact, the prominent parts of the 
one will often fall into the hollow parts of the other, and 
occasion more or less resistance to motion. 

Caroline. But if a machine is made of polished metal, 
as a watch, for instance, the friction must be very trifling ? 

Mrs. B. In proportion as the surfaces of bodies are 
well polished, the friction is doubtless diminished ; but it 
is always considerable, and it is usually computed to de- 
stroy one-third of the power of a machine. Oil or grease is 
used to lessen friction ; it acts as a polish by filling up the 
cavities of the rubbing surfaces, and thus making them 
slide more easily over each other. 

Caroline. Is it for this reason that wheels are greased, 
and the locks and hinges of doors oiled ? 

Mrs. B. Yes ; in these instances the contact of the 
rubbing surfaces is so close, and the rubbing so continual, 
thai notwithstanding their being polished and oiled, a con- 
siderable degree of friction is produced. 

There are two kinds of friction ; the one occasioned 
by the sliding of the flat surface of a body, the other by 
the rolling of a circular body; the friction resulting from 
the first is much the most considerable, for great force 
is required to enable the sliding body to overcome 

2G6. What diminishes the force of all machinery ? 

667. What are we to understand by friction in machinery ? 

268. In what proportion is the friction of machinery destroyed ? 

209. How much of the power of a machine is reckoned to 

be destroyed by friction ? 270. What is commonly used to 

lessen the friction of machinery ? 271. Why will oil and grease 

lessen the friction of machinery ? 272. How many kinds of 

friction are there ? 273. What are they ? 274. Which is 

the most considerable '" 



ON THE MECHANICAL POWERS. H 

the resistance which the asperities of the surfaces in con- 
tact oppose to its motion, and it must be either lifted over 
or break through them ; whilst, in the other kind of fric- 
tion, the rough parts roll over each other with comparative 
facility ; hence it is, that wheels are often used for the 
sole purpose of diminishing the resistance of friction. 

Emily. This is one of the advantages of carriage- 
wheels ; is it not ? 

Mrs. B. Yes ; and the larger the circumference of 
the wheel, the more readily it can overcome any consider- 
able obstacles, such as stones, or inequalities in the road. 
When, in descending a steep hill, we fasten one of the 
wheels, we decrease the velocity of the carriage, by in- 
creasing the friction. 

Caroline. That is to say, by converting the rolling fric- 
tion into the dragging friction. And when you had casters 
put to the legs of the table, in order to move it more easily, 
you changed the dragging into the rolling friction. 

Mrs. B. There is another circumstance which we 
have already noticed, as diminishing the motion of bodies, 
and which greatly affects the power of machines. This 
is the resistance of the medium in which a machine is 
worked. All fluids, whether of the nature of air, or of 
water, are called mediums ; and their resistance is pro- 
portioned to their density ; for the more matter a body 
contains, the greater the resistance it will oppose to the 
motion of another body striking against it . 

Emily. It would then be much more difficult to work 
a machine under water than in the air ? 

Mrs. B. Certainly, if a machine could be worked in 
vacuo, and without friction, it would be perfect ; but this 
is unattainable : a considerable reduction of power must 
therefore be allowed for the resistance of the air. 

We shall here conclude our observations on the me- 
chanical powers. At our next meeting I shall endeavour 
to give you an explanation of the motion of the heavenly 
bodies. 



275. Which will most readily overcome obstacles, a large or a 

small wheel ? 276. Why is a wheel fastened on descending 

a hill ? 277. What besides friction diminishes the force of all 

machinery ? 278. What is meant by mediums ? 279. To 

what is their resistance proportioned ? 280. In what state 

would the force of machinery be perfect ? 



78 CAUSES OF THE EARTIl's ANNUAL MOTION. 

CONVERSATION VI. 

CAUSES OF THE EARTIl's ANNUAL MOTION. 

Of the Planets, and their Motion ; Of the Diurnal Mo- 
tion of the Earth and Planets. 

CAROLINE. 

I am come to you to-day quite elated with the spirit of 
opposition, Mrs. B. ; for I have discovered such a pow- 
erful objection to your theory of attraction, that I doubt 
whether even your conjuror Newton, with his magick 
wand of attraction, will be able to dispel it. 

Mrs. B. Well, my dear, pray what is this weighty 
objection ? 

Caroline. You say that bodies attract in proportion to 
the quantity of matter they contain : now we all know the 
sun to be much larger than the earth ; why, therefore, 
does it not attract the earth ; you will not, I suppose, pre- 
tend to say that we are falling towards the sun 7 

Emily. However plausible your objection appears, 
Caroline, I think you place too much reliance upon it : 
when any one has given such convincing proofs of saga- 
city and wisdom as Sir Isaac Newton, when we find that 
his opinions are universally received and adopted, is it to 
be expected that any objection we can advance should 
overturn them ? 

Caroline. Yet I confess that I am not inclined to 
yield implicit faith even to opinions of the great Newton : 
for what purpose are we endowed with reason, if we are 
denied the privilege of making use of it, by judging for 
ourselves ? 

Mrs. B. It is reason itself which teaches us, that 
when we, novices in science, start objections to theories 
established by men of acknowledged wisdom, we should 
be diffident rather of our own than of their opinion. I am 
far from wishing to lay the least restraint on your ques- 
tions ; you cannot be better convinced of the truth of a 
system, than by finding that it resists all your attacks ; 
but I would advise you not to advance your objections 

281. If bodies attract each other in proportion to the quantity 
of matter they contain, why does not the sun attract the earth 
completely to itself? 



CAUSES OF THE EARTH'S ANNUAL MOTION. 79 

with so much confidence, in order that the discovery of 
their fallacy may be attended with less mortification. 
In answer to that you have just proposed, I can only say, 
that the earth really is attracted by the sun. 

Caroline. Take care at least that we are not consum- 
ed by him, Mrs. B. 

Mrs. B. We are in no danger ; but our magician 
Newton, as you are pleased to call him, cannot extricate 
himself from this difficulty without the aid of some caba- 
listical figures, which I must draw for him. 

Let us suppose the earth, at its creation, to have been 
projected forwards into universal space : we know that 
if no obstacle impeded its course, it would proceed in the 
same direction, and with a uniform velocity for ever. In 
fig. 1, plate 6., A represents the earth, and S the sun. 
We shall suppose the earth to be arrived at the point in 
which it is represented in the figure, having a velocity 
which would carry it on to B in the space of one month ; 
whilst the sun's attraction would bring it to C in the same 
space of time. Observe that the two forces of projection 
and attraction do not act in opposition, but perpendicu- 
larly, or at a right angle to each other. Can you tell me 
now, how the earth will move ? 

Emily. I recollect your teaching us that a body act- 
ed upon by two forces perpendicular to each other would 
move in the diagonal of a parallelogram ; if, therefore, I 
complete the parallelogram by drawing the lines C D, 
B D, the earth will move in the diagonal A D. 

Mrs. B. A ball struck by two forces acting perpen- 
dicularly to each other, it is true, moves in the diagonal 
of a parallelogram ; but you must observe that the force of 
attraction is continually acting upon our terrestrial ball, 
and producing an incessant deviation from its course in 
a right line, which converts it into that of a curve line ; 
every point of which may be considered as constituting 
the diagonal of an infinitely small parallelogram. 

^ 282. If the earth at its creation had been put in motion by a 
single force without resistance, what would have been its course ? 

283. How would you illustrate this by the figure ? 284. 

What prevents the earth from proceeding on in a right line, as im- 
pelled by its projectile force ? 285. In what direction does the 

attraction of the sun operate on the projectile force of the earth ? 

28& When two forces ^operate perpendicularly on each 

other, in what direction will be their compound motion ? 287. 

Why then is the line A D in Figure 1, circular instead of being a 
right line diagonal to the parallelogram. ABDC? 



80 CAUSES OF THE EARTIl's ANNUAL MOTION. 

Let us detain the earth a moment at the point D, and 
consider how it will be affected by the combined action of 
the two forces in its new situation. It still retains its ten- 
dency to fly off in a straight line ; but a straight line 
would now carry it away to F, whilst the sun would at- 
tract it in the direction D S ; how then will it proceed ? 

Emily. It will go on in a curve line, in a direction 
between that of the two forces. 

Mrs, B. In order to know exactly what course the 
earth will follow, draw another parallelogram similar to 
the first, in which the line D F describes the force of pro- 
jection, and the line D S, that of attraction ; and you will 
find that the earth will proceed in the curve line D G. 

Caroline. You must now allow me to draw a parallel- 
ogram, Mrs. B. Let me consider in what direction will 
the force of projection now impel the earth. 

Mrs. B. First draw a line from the earth to the sun 
representing the force of attraction : then describe the 
force of projection at a right angle to it. 

Caroline. The earth will then move in the curve G I, 
of the parallelogram G H I K. 

3Irs. B. You recollect that a body acted upon by 
two forces, moves through a diagonal in the same time 
that it would have moved through one of the sides of the 
parallelogram, were it acted upon by one force only. 

The earth has passed through the diagonals of these 
three parallelograms in the space of three months, and 
has performed one quarter of a circle ; and on the same 
principle it will go on till it has completed the whole of 
the circle. It will then recommence a course, which it 
has pursued ever since it first issued from the hand of its 
Creator, and which there is every reason to suppose it 
will continue to follow, as long as it remains in existence. 

Emily. What a grand and beautiful effect resulting 
from so simple a cause ! 

Caroline. It affords an example on a magnificent 
scale, of the circular motion which you taught us in 
mechanicks. The attraction of the sun is the centripetal 
force, which confines the earth to a centre ; and the im- 



288. How would you explain the continued motion of the earth 

about the sun by the use of Fig. 1, plate VI ? 289. What is the 

attraction of the sun called ? 290. And what is the projectile 

force of the cartli called ? 



CAUSES OF THE EARTH'S ANNUAL MOTION. 81 

pulse of projection the centrifugal force, which impels 
the earth to quit the sun and fly off in a tangent. 

Mrs. B. Exactly so. A simple mode of illustrating 
the effect of these combined forces on the earth, is to cut 
a slip of card in the form of a right angle, (fig. 2, plate 
VI.) to describe a small circle at the angular point re- 
presenting the earth, and to fasten the extremity of one of 
the legs of the angle to a fixed point, which we shall con- 
sider as the sun. Thus situated, the angle will represent 
both the centrifugal and centripetal forces; and if you 
draw it round the fixed point, you will see how the di- 
rection of the centrifugal force varies, constantly forming 
a tangent to the circle in which the earth moves, as it is 
constantly at a right angle with the centripetal force. 

Emily. The earth, then, gravitates towards the sun 
without the slightest danger either of approaching nearer 
or receding further from it. How admirably this is con- 
trived ! If the two forces which produce this circular mo- 
tion had not been so accurately adjusted, one would ulti- 
mately have prevailed over the other, and we should either 
have approached so near the sun as to have been burnt, 
or have receded so far from it as to have been frozen. 

Mrs. B. What will you say, my dear, when I tell 
you that these two forces are not, in fact, so proportion- 
ed as to produce circular motion in the earth ? 

Caroline. You must explain to us, at least, in what 
manner we avoid the threatened destruction. 

Mrs. B. Let us suppose that when the earth is at 
A. (fig. 3.), its projectile force should not have given it 
a velocity sufficient to counterbalance that of gravity, so 
as to enable these powers conjointly to carry it round the 
sun in a circle ; the earth, instead of describing the line 
A C, as in the former figure, will approach nearer the sun 
in the line A B. 

Caroline. Under these circumstances, I see not what 
is to prevent our approaching nearer and nearer the sun 
till we fall into it : for, its attraction increases as we ad- 
vance towards it, and produces an accelerated velocity in 
the earth, which increases the danger. 

291. What simple illustration is given in Fig. 2, plate VI. of 
the combined forces, which produced the revolution of the earth 
about the sun ? 292. Does the earth revolve in an exact cir- 
cle about the sun ? 293. What is the design of Fig. 3, plate 

VI. r 294. In Fig. 3, plate VI. why is the earth in the line at 

B instead of the line at C according to the principle of Fig. 1. ? 



82 CAUSES OP THE EARTH'S ANNUAL MOTION. 

Mrs. B. And there is yet another danger, of which 
you are not aware. Observe, that as the earth approaches 
the sun, the direction of its projectile force is no longer per- 
pendicular to that of attraction, but inclines more nearly 
to it. When the earth reaches that part of its orbit at B, 
the force of projection would carry it to D, which brings 
it nearer the sun instead of bearing it away from it. 

Emily. If, then, we are driven by one power and 
drawn by the other to this centre of destruction, how is 
it possible for us to escape 1 

Mrs. B. A little patience, and you will find that we 
are not without resource. The earth continues approach- 
ing the sun with a uniformly increasing accelerated mo- 
tion, till it reaches the point E. In what direction will 
the projectile force now impel it 1 

Emily. In the direction E F. Kere then the two forces 
act perpendicularly to each other, and the earth is situat- 
ed just as it was in the preceding figure ; therefore, from 
this point, it should revolve round the sun in a circle. 

Mrs. B. No, all the circumstances do not agree. 
In motion round a centre, you recollect that the centri- 
fugal force increases with the velocity of the body, or, in 
other words, the quicker it moves, the stronger is its ten- 
dency to fly off in a right line. When the earth, there- 
fore, arrives at E, its accelerated motion will have so 
far increased its velocity, and consequently its centrifugal 
force, that the latter will prevail over the force of at- 
traction, and drag the earth away from the sun till it 
reaches G. 

Caroline. It is thus, then, that we escape from the 
dangerous vicinity of the sun ; and in proportion as we 
recede from it, the force of its attraction, and, conse- 
quently, the velocity of the earth's motion are dimi- 
nished. 

Mrs. B. Yes. From G the direction of projection is 
towards II, that of attraction towards S, and the earth 
proceeds between them with a uniformly retarded motion, 
till it has completed its revolution. Thus you see, that 
the earth travels round the sun, not in a circle, but an 



295. When the earth arrives at E in the figure, why does it 
not revolve in a small circular orbit instead of receding off in the 

direction G ? 296. What is the figure called that the earth de- 

fcrribps in its revolution about the sun? 



CAUSES OF THE EARTH 's ANNUAL MOTION. 83 

ellipsis, of which the sun occupies one of the foci ; and 
that in its course the earth alternately approaches, and 
recedes from it, without any danger of being either swal- 
lowed up, or being entirely carried away from it. 

Caroline, And I observe, that what I apprehended 
to be a dangerous irregularity, is the means by which the 
most perfect order and harmony are produced ! 

Emily. The earth travels, then, at a very unequal 
rate, its velocity being accelerated as it approaches the 
sun, and retarded as it recedes from it. 

Mrs. B. It is mathematically demonstrable, that, in 
moving round a point towards which it is attracted, a body 
passes over equal areas in equal times. The whole of the 
space contained within the earth's orbit, is in fig. 4., di- 
vided into a number of areas, or spaces, 1, 2, 3, 4, &c. all 
of which are of equal dimensions, though of very different 
forms ; some of them, you see, are long and narrow, others 
broad and short : but they each of them contain an equal 
quantity of space. An imaginary line drawn from the cen- 
tre of the earth to that of the sun, and keeping pace with 
the earth in its revolution, passes over equal areas in equal 
times ; that is to say, if it is a month going from A to B, it 
will be a month going from B to C, and another from C 
to E, and so on. 

Caroline. What long journeys the earth has to per- 
form in the course of a month, in one part of her orbit, 
and how short they are in the other part ! 

Mrs. B. The inequality is not so considerable as ap- 
pears in this figure ; for the earth's orbit is not so eccen- 
trick as it is there described ; and, in reality differs but 
little from a circle ; that part of the earth's orbit nearest the 
sun is called its Perihelion, that part most distant from the 
sun its Aphelion ; and the earth is above three millions of 
miles nearer the sun at its perihelion than at its aphelion. 



297. What is the name of the place occupied by the sun with- 
in the orbit of the earth ? 298. Is the earth's motion in moving 

round the sun uniform ? 299. What is mathematically demon- 
strable in relation to a body moving round a point towards which it 

is attracted ? #00. What is the design of Fig. 4, plate VI. ? 

301. What is that part of the earth's orbit called which is most dis- 
tant from the sun ? 302. What is that part called which is 

nearest the sun ? 303. How much nearer is the earth to the sun 

in perihelion than at its aphelion ? 



84 causes or the earth's ANNUAL MOTION. 

Emily. I think I can trace a consequence from these 
different situations of the earth ; is it not the cause of 
summer and winter ? 

Mrs. B. On the contrary ; during the height of sum- 
mer, the earth is in that part of its orbit which is most 
distant from the sun, and it is during the severity of win- 
ter, that it approaches nearest to it. 

Emily. That is very extraordinary ; and how then 
do you account for the heat being greatest, when we are 
most distant from the sun ? 

Mrs. B. The difference of the earth's distance from 
the sun in summer and winter, when compared with its total 
distance from the sun, is but inconsiderable. The earth, it 
is true, is above three millions of miles nearer the sur. 
winter than in summer ; but that distance, however grea. 
at first appears, sinks into insignificance in comparison ^ r 
95 millions of miles, which is our mean distance from t 
sun. The change of temperature, arising from this difi 
rence, would scarcely be sensible, were it not complete 
overpowered by other causes which produce the variation, 
of the seasons ; but these I shall defer explaining till we havei 
made some further observations on the heavenly bodies. 

Caroline. And should not the sun appear smaller in 
summer, when it is so much further from us 1 

Mrs. B. It actually does when accurately measured 
but the apparent difference in size, is, I believe, not per- 
ceptible to the naked eye. 

Emily. Then, since the earth moves with the greates 
velocity in that part of its orbit nearest the sun, it musi 
have completed its journey through one half of its orbit ii 
a shorter time than the other half? 

Mrs. B. Yes, it is about seven days longer perform- 
ing the summer-half of its orbit, than the winter-half. 
The revolution of all the planets round the sun is the re- 
sult of the same causes, and is performed in the same 
manner as that of the earth. 

Caroline. Pray what are the planets 1 

Mrs. B. They are those celestial bodies, which re- 
volve like our earth about the sun ; they are supposed toi 
resemble the earth also in many other respects ; and wei 

304. Is the earth nearest the sun in summer or winter ? 305. 

Diuch longer is the earth performing the summer-h?!lf 
the m 306. VVh 



J 



CAUSES OF THE EARTH'S ANNUAL MOTION. 85 

are led by analogy to suppose them to be inhabited 
worlds. 

Caroline. I have heard so ; but do you not think suck 
an opinion too great a stretch of the imagination ? 

Mrs. B. Some of the planets are proved to be larger 
than the earth ; it is only their immense distance from us, 
which renders their apparent dimensions so small. Now' 
if we consider them as enormous globes, instead of small 
^twinkling spots, we shall be led to suppose, that the Al- 
mighty would not have created them merely for the pur- 
pose of giving us a little light in the night, as it was 
formerly imagined, and we should find it more consistent 
with our ideas of the Divine wisdom and beneficence to 
appose that these celestial bodies should be created for 
ie habitation of beings, who are, like us, blessed by his 
^ovidence. Both in a moral as well as a physical point 
f view, it appears to me more rational to consider the 
lanets as worlds revolving round the sun ; and the fixed 
.are as other suns, each of them attended by their re- 

i^pective system of planets, to which they impart their in- 
tueoce. We have brought our telescopes to such a de- 
ree of perfection, that from the appearances .which the 
loon exhibits when seen through them, we have very 
*ood reason to conclude, that it is a habitable globe, for 
hough it is true, that we cannot discern its towns and 
people, we can plainly perceive its mountains and val- 
eys ; and some astronomers have gone so far as to ima- 
gine they discovered volcanoes. 

Emily. If the fixed stars are suns, with planets re- 
volving round them, why should we not see those planets 
Is well as their suns ? 

. Mrs. B. In the first place, we conclude that the 
-Janets of other systems, (like those of our own,) are much 
Waller than the suns which give them light ; therefore 
t so great a distance as to make the suns appear like 
xed stars, the planets would be quite invisible. Second- 
U the light of the planets being only reflected light, is 
men more feeble than that of the fixed stars. There is 
xactly th ejsa me diff erence as between the light of the 

VKL i Why d ° We su PP ose th e planets are inhabited ? 308. 

• n y t Me TM ld3 li£e ourowi Mvliv do they appear so 
\ TLZTa iu , tlie , tlxed stars are suns, with planets revolv- 

! 'Ilir suns ? ' * Sh ° Uld ** not ^ ee tli0£e P laneta as *•» ■■ 

S 



86 CAU8K8 OF THE EARTIl's ANNUAL MOTION* 

sun and that of the moon ; the first being a fixed star, the 
second a planet. 

Emily. But if the planets are worlds like our earth, 
they are dark bodies ; and instead of shining by night, 
we should see them only by day-light. And why do we 
not see the fixed stars also by day-light ? 

Mrs. B. Both for the same reason ; their light is so 
faint, compared to that of our sun reflected by the atmo- 
sphere, that it is entirely effaced by it : the light emitted 
by the fixed stars may probably be as strong as that of our 
sun, at an equal distance ; but being so much more remote, 
it is diffused over a greater space, and is consequently 
proportionally weakened. 

Caroline. True ; I can see much better by the light of a 
candle that is near me,than by that of one at a great distance. 
But I do not understand what makes the planets shine 1 

Mrs. B. What is it that makes the steel buttons on 
your brother's coat shine ? 

Caroline. The sun. But if it was the sun which 
made the planets shine, we should see them in the day- 
time when the sun shone upon them ; or if the faintness 
of their light prevented our seeing them in the day, we 
should not see them at all, for the sun cannot shine upon 
them in the night. 

Mrs. B. There you are in error. But in order to 
explain this to you, I must first make you acquainted with 
the various motions of the planets. 

You know, that according to the laws of attraction, the 
planets belonging to our system all gravitate towards the 
sun ; and that this force combined with that of projection, 
will occasion their revolution round f he sun, in orbits more 
or less elliptical, according to the proportion which these 
two forces bear to each other. 

But the planets have also another motion ; they re- 
volve upon their axes. The axis of a planet is an ima- 
ginary line which passes through its centre, and on which 
it turns ; and it is this motion which produces day and 
night. With that side of the planet facing the sun it is 
day ; and with the opposite side, which remains in dark- 
ness, it is night. Our earth, which we consider as a 
planet, is 24 hours in performing one revolution on its 

310. Why do we not see tho stars in the daytime? 311 

What motion have the planets besides that about the sun? 312 

What is the axis of a planet ? 






CAUSES OF THE EARTH 's ANNUAL MOTION. 87 

axis ; in that period of time, therefore, we have a day and 
a night ; hence this revolution is called the earth's diur- 
nal or daily motion ; and it is this revolution of the earth 
from west to east which produces an apparent motion of 
the sun, moon, and stars in a contrary direction. 

Let us now suppose ourselves to be beings independ- 
ent of any planet, travelling in the skies, and looking up- 
on the earth in the same point of view as upon the other 
planets. 

Caroline. It is not flattering to us, its inhabitants, to 
see it make so insignificant an appearance. 

Mrs. B. To those who are accustomed to contem- 
plate it in this light, it never appears more glorious. 
We are taught by science to distrust appearances : and 
instead of considering the planets as little stars, we look 
upon them either as brilliant suns or habitable worlds, 
and we consider the whole together as forming one vast 
and magnificent system, worthy of the Divine hand by 
which it was created. 

Emily. I can scarcely conceive the idea of this im- 
mensity of creation ; it seems too sublime for our ima- 
gination : — and to think that the goodness of Providence 
extends over millions of worlds throughout a boundless 
universe — Ah ! Mrs. B., it is we only who become trifling 
and insignificant beings in so magnificent a creation. 

Mrs. B. This idea should teach us humility, but with- 
out producing despondency. The same Almighty hand 
which guides these countless worlds in their undeviating 
course, conducts with equal perfection the blood as it cir- 
culates through the veins of a fly, and opens the eye of 
the insect to behold His wonders. Notwithstanding this 
immense scale of creation, therefore, we need not fear to 
be disregarded or forgotten. 

But to return to our station in the skies. We were, 
if you recollect, viewing the earth at a great distance, in 
appearance a little star, one side illuminated by the sun, 
the other in obscurity. But would you believe it, Ca- 
roline, many of the inhabitants of this little star imagine 
that when that part which they inhabit is turned from the 
sun, darkness prevails throughout the universe merely 



313. What are we taught by science ? 314. If die planets 

are only seen by the reflected light of the sun, how is it that th?y 
ran hp PPfin in thp rntrht ? 



can be seen in the night ? 



88 CAUSES OF THE EARTH^S ANNUAL MOTION. 

because it is night with them ; whilst, in reality, the sun 
never ceases to shine upon every planet. When, there- 
fore, these little ignorant beings look around them during 
their night, and behold all the stars shining, they cannot 
imagine why the planets, which are dark bodies, should 
shine, concluding, that since the sun does not illumine 
themselves, the whole universe must be in darkness. 

Caroline. I confess that I was one of these ignorant 
people ; but I am now very sensible of the absurdity of 
such an idea. To the inhabitants of the other planets, 
then, we must appear as a little star ? 

Mrs. B. Yes, to those which revolve round our sun ; 
for since those which may belong to other systems (and 
whose existence is only hypothetical,) are invisible to us, 
it is probable, that we also are invisible to them. 

Emily. But they may see our sun as we do theirs, in 
appearance a fixed star 1 

Mrs. B. No doubt, if the beings who inhabit those 
planets are endowed with senses similar to ours. By the 
same rule, we must appear as a moon, to the inhabitants 
of our moon ; but on a larger scale, as the surface of the 
earth is about thirteen times as large as that of the moon. 

Emily. The moon, Mrs. B., appears to move in a 
different direction, and in a different manner from the 
stars 1 

Mrs. B. I shall defer the explanation of the motion 
of the moon, till our next interview, as it would prolong 
our present lesson too much. 

315. How must the earth appear to the inhabitants of other 

planets ? 316. How much larger does the earth appear viewed 

at the moon, than the moon appears viewed at the earth ? 



ON THE PLANETS. 89 

CONVERSATION VII. 

ON THE PLANETS. 

Of the Satellites or Moons ; Gravity diminishes as the 
Square of the distance ; Of the Solar System ; Of Co- 
mets ; Constellations, Signs of the Zodiaclc ; Of Co- 
pernicus, Neivton, Spc. 

MRS. B. 

The planets are distinguished into primary and secon- 
dary. Those which revolve immediately about the sun 
are called primary. Many of these are attended in their 
course by smaller planets, which revolve round them : 
these are called secondary planets, satellites, or moons. 
Such is our moon which accompanies the earth, and is 
carried with it round the sun. 

Emily. How then can you reconcile the motion of the 
secondary planets to the laws of gravitation ; for the sun 
is much larger than any of the primary planets ; and is 
not the power of gravity proportional to the quantity of 
matter 1 

Caroline. Perhaps the sun, though much larger, may 
be less dense than the planets. Fire you know is very 
light, and it may contain but little matter though of great 
magnitude. 

Mrs. B. We do not know of what kind of matter the 
sun is made ; but we may be certain, that since it is the 
general centre of attraction of our system of planets, it 
must be the body which contains the greatest quantity of 
matter in that system. 

You must recollect, that the force of attraction is not 
only proportional to the quantity of matter, but to the 
degree of proximity of the attractive body : this power is 
weakened by being diffused, and diminishes as the squares 
of the distances increase. The square is the product of 

317. How are the planets distinguished? 318. What are 

the primary planets ? 319. What are the secondary planets ? 

320. Ey what other names are the secondary planets called ? 

321. To what is the force of attraction proportional besides 

the quantity of matter in the attracting bodies ? 322. What 

is meant hv the square of distance ? 

8* 



90 ON THE PLANETS. 

a number multiplied by itself; so that a planet situated at 
twice the distance at which we are from the sun would 
gravitate four times less than we do ; for the product of 
two multiplied by itself is four. 

Caroline. Then the more distant planets move slower 
in their orbits ; for their projectile force must be propor- 
tioned to that of attraction ? But I do not see how this 
accounts for the motion of the secondary round the pri- 
mary planets, in preference to the sun. 

Emily. Is it not because the vicinity of the primary 
planets renders their attraction stronger than that of the 
sun. 

Mrs. B. Exactly so. But since the attraction be- 
tween bodies is mutual, the primary planets are also at- 
tracted by the satellites, which revolve round them. The 
moon attracts the earth, as well as the earth the moon ; 
but as the latter is the smaller body, her attraction is pro- 
portionally less ; therefore neither the earth revolves round 
the moon, nor the moon round the earth ; but they both 
revolve round a point, which is their common centre of 
gravity, and which is as much nearer the earth than the 
moon, as the gravity of the former exceeds that of the 
latter. 

Emily. Yes, I recollect your saying, that if two bodies 
were fastened together by a wire or bar, their common 
centre of gravity would be in the middle of the bar, pro- 
vided the bodies were of equal weight ; and if they differed 
in weight, it would be nearer the larger body. If then 
the earth and moon had no projectile force which pre- 
vented their mutual attraction from bringing them to- 
gether, they would meet at their common centre of gravity. 

Caroline. The earth then has a great variety of mo- 
tions, it revolves round the sun, upon its axis, and round 
the point towards which the moon attracts it. 

Mrs. JB. Just so ; and this is the case with every 
planet which is attended by satellites. The complicated 
effect of this variety of motions, produces certain irregu- 
larities, which, however, it is not necessary to notice at 
present. 

323. How much less does a planet gravitate towards the sun 
than the earth, at twice the distance of the earth from the sun ? 

— 324. Why does not the sun attract the secondary planets 
from their primaries ? 325. What motion has the earth be- 
sides that about the sun and on its own axis ? 326. Where is 

the common centre of gravity to the sun and moon ? 



ON THE PLANETS. 91 

The planets act on the sun in the same manner as they 
are themselves acted on by their satellites ; for attraction, 
you must remember, is always mutual ; but the gravity of 
the planets (even when taken collectively) is so trifling 
compared with that^of the sun, that they do not cause the 
latter to move so much as one half of his diameter. The 
planets do not, therefore, revolve round the centre of the 
sun, but round a point at a small distance from its centre, 
about which the sun also revolves. 

Emily. I thought the sun had no motion ? 

Mrs. B. You were mistaken ; for besides that which 
I have just mentioned, which is indeed very inconsidera- 
ble, he revolves on his axis ; this motion is ascertained 
by observing certain spots which disappear, and re-appear 
regularly at stated times.* 



* The sun is a spherical body, situated near the centre of gravi- 
ty in the system of planets, of which our earth is one. Its dia- 
meter is 877,547 English miles ; or equal to 100 diameters of the 
earth ; and therefore its cubick magnitude must exceed that of the 
earth one million of times. It revolves round its axis in 25 days, 
and 10 hours, which has been determined by means of several dark 
spots seen with telescopes on that luminary. Dr. Herschel sup- 
poses these spots in the sun to be the appearance of the opaque 
body of the sun through the openings in his luminous atmosphere. 

Its similarity to the other globes of the solar system, in solidity, 
atmosphere, surface diversified with mountains and valleys, and 
rotation on its axis, lead us to suppose, that it is most probably in- 
habited like the rest of the planets, by beings whose organs are 
adapted to their peculiar circumstances. 

Though it may be objected, from the effects produced at the 
distance of 95,000,000 miles, that every thing must be scorched up 
at its surface, yet many facts show that heat is produced by the 
sun's rays only when they act on a suitable medium ; or when 
radiated and reflected by suitable surfaces. On the tops of moun- 
tains of sufficient height, we always find regions of ice and snow ; 
though if the solar rays themselves conveyed all the heat we find 
on this globe, it ought to be hottest where their course is the least 
interrupted. 



327. Do the planets revolve round the centre of the sun ? 

328. Around what do they revolve ? 329. Has the sun any 

motion ? 330. How is it known that the sun turns on its axis ? 

331. How much greater is the diameter of the sun than of 

the earth ? 332. How much does hbs cubick magnitude exceed 

that of the earth 9 333. What does Dr. Herschel suppose 

the dark spots on the sun's disk to be ? 334. What are we led to 

suppose from the similarity of the sun to the other globes of the 
solar system f 



92 ON THE PLANETS. 

Caroline. A planet has frequently been pointed out to 
me in the heavens ; but I could not perceive that its mo- 
tion differed from that of the fixed stars, which only appear 
to move. 

Mrs. B. The great distance of the planets renders 
their motion apparently so slow, that the eye is not sen- 
sible of their progress in their orbit, unless we watch them 
for some considerable length of time : in different seasons 
they appear in different parts of the heavens. The most 
accurate idea I can give you of the situation and motion 
of the planets, will be by the examination of this diagram, 
(plate VII. fig. 1.) representing the solar system, in which 
you will find every planet with its orbit delineated. 

Emily. But the orbits here are all circular, and you said 
that they were elliptical. The planets appear too, to be 
moving round the centre of the sun ; whilst you told us that 
they moved round a point at a little distance from thence. 

Mrs. B. The orbits of the planets are so nearly cir- 
cular, and the common centre of gravity of the solar sys- 
tem so near the centre of the sun, that these deviations 
are scarcely worth observing. The dimensions of the 
planets, in their true proportions, you will find delineated 
in fig. 2. 

Mercury is the planet nearest the sun ; his orbit is con- 
sequently contained within ours; but his vicinity to the 
sun occasions his being nearly lost in the brilliancy of 
his rays ; and when we see the sun, he is so dazzling 
that very accurate observations cannot be made upon 
Mercury. He performs his revolution round the sun in 
about 87 days, which is consequently the length of his 
year. The time of his rotation on his axis is not known ; 
his distance from the sun is computed to be 37 millions 
of miles, and his diameter 3180 miles. The heat of this 
planet is so great, that water cannot exist there, but in a 
state of vapour, and metals would be liquefied.* 

* The intenseness of the sun's heat, which is in the same pro- 
portion as his light, is seven times as great in Mercury as with us \ 

335. Can the motion of the planets be seen by the naked eye ? 

336. What is the design of Fig. 1, plate VII ? 337. 

Which figure exhibits the dimensions of the planets in their truo 

Proportions ? 338. What planet is nearest the sun ? 33i). 
n what time does Mercury revolve round the sun ? 340. What 

is his distance from the sun ? 341. What is his diameter ? 

'M2. How docs the intenseness of the sun's heat at Mercury com- 
pare with it at our earth-' 



ON THE PLANETS. 93 

Caroline. Oh, what a dreadful climate. 

Mrs. B. Though we could not live there, it may be 
perfectly adapted to other beings destined to inhabit it. 

Venus, the next in the order of planets, is 63 millions 
of miles from the sun ; she revolves about her axis in 23 
hours and Z\ minutes, and goes round the sun in 244 
days 17 hours. The orbit of Venus is also within ours ; 
during one half of her course in it, we see her before sun- 
rise, and she is called the morning star ; in the other part 
of her orbit, she rises later than the sun.* 

Caroline. In that case, we cannot see her, for she 
must rise in the day time ? 

Mrs. B. True ; but when she rises later than the sun, 
she also sets later ; so that we perceive her approaching 
the horizon after sun-set : she is then called Hesperus, or 
the evening star. Do you recollect those beautiful lines 
of Milton ? 

Now came still evening on, and twilight gray 
Had in her sober livery all things clad : 
Silence accompanied ; for beast and bird, 
They to their grassy couch, these to their nests 
Were slunk, all but the wakeful nightingale ; 
She all night long her amorous descant sung ; 
Silence was pleas'd ; now glow'd the firmament 
With living sapphires. Hesperus, that led 
Th3 starry host, rode brightest, till the moon 
Rising in clouded majesty, at length 
Apparent queen unveil'd her peerless light, 
And o'er the dark her silver mantle threw. 



so that water there would be carried off in the shape of steam, for 
by experiments with the thermometer, it appears that a heat seven 
times greater than that of the sun's beams in summer will serve to 
make water boil. 

* In most treatises on Astronomy, Mercury and Venus are call- 
ed infer lour, and those more distant from the sun than our earth, 
superiour planets ; but, it is considered a more proper distinction, 
to call the former interiour and the latter exteriour planets. 

343. How much greater heat is required to make water boil, 

than that of the sun in summer at the earth ? 344. How far 

is Venus from the sun ? 345. In what time does it revolve 

round the sun ? 34G. In what time does it revolve upon its 

axis ? 347. When is Venus called the morning and when the 

evening star ? 348. By ichat nume hare Mercury and Venus 

usually been distinguished from the other planets ? 349. How 

should the planets more distant, and those less distant from the 
sun than the earth, be distinguished from each other ? 



94 ON THE PLANETS. 

The planet next to Venus is the Earth, of which we 
shall soon speak at fill'- length. At present I shall only 
observe, that we are 95 millions of miles distant from the 
sun, that we perform our annual revolution in 365 days, 
5 hours, and 49 minutes ; and are attended in our course 
by a single moon. 

Next follows Mars. He can never come between us 
and the sun, like Mercury and Venus ; his motion is, 
however, very perceptible, as he may be traced to differ- 
ent situations in the heavens ; his distance from the sun 
is 144 millions of miles ; he turns round his axis in 24 
hours and 39 minutes ; and he performs his annual revo- 
lution in about 687 of our days : his diameter is 4120 
miles. Then follow four very small planets, Juno, Ce- 
res, Pallas, and Vesta, which have been recently disco- 
vered, but whose dimensions and distances from the sun 
have not been very accurately ascertained.* 

Jupiter is next in order : this is the largest of all the 
planets. He is about 490 millions of miles from the sun, 
and completes his annual period in nearly 12 of our years. 
He turns round his axis in about ten hours. He is above 
1200 times as big as our earth ; his diameter oeing 86,000 
miles. The respective proportions of the planets cannot, 
therefore, you see, be conveniently delineated in a dia- 
gram. He is attended by four moons.f 

* These anomalous bodies, so unlike the other primary planets. 
Dr. Herschel has denominated Asteroids. Probably they are the 
fragments of some planet ; or perhaps other similar bodies abound 
in the solar system, though they have hitherto, from their small- 
ness or darkness, escaped observation. 

t Jupiter is surrounded by cloudy substances, subject to fre 
quent changes in their situation and appearance, called Belts 
These Belts are sometimes of a regular form ; sometimes inter 
rupted and broken ; and sometimes not at all to be seen. 



350. How far distant from the sun is the earth ? 351. In 

what time does it revolve round the sun ? 352. Which planet 

is next to the earth in distance from the sun. 353. How far 

is Mars from the sun ? 354. How long time is occupied in his 

revolution about the sun ? 355. What four small planets are 

next to Mars in distance from the sun ? 356. What did Dr. 

Herschel call these planets? 357. What is the distance of 

Jupiter from the sun ? 358. In what time does Jupiter com- 
plete his revolution ? 359. How much larger is Jupiter than 

our earth ? 360. How manv satellites has this planet ?»■— « 

361. By what is Jupiter surrounded ? 



ON THE PLANETS. 95 

The next planet is Saturn, whose distance from the sun 
is about 900 millions of miles ; his diurnal rotation is per- 
formed in 10 hours and a quarter : — his annual revolution 
in nearly 30 of our years. His diameter is 79,000 miles. 
This planet is surrounded by a luminous ring, the nature 
of which, astronomers are much at a loss to conjecture ; 
he has seven moons.* Lastly, we observe the Georgium 
Sidus, discovered by Dr. Herschel, and which is attended 
by six moons. 

Caroline. How charming it must be in the distant 
planets, to see several moons shining at the same time ; 
I think I should like to be an inhabitant of Jupiter or 
Saturn. 

Mrs, B. Not long, I believe. Consider w r hat ex- 
treme cold must prevail in a planet, situated as Saturn is, 
at nearly ten times the distance at which we are from the 
sun. Then his numerous moons are far from making so 
splendid an appearance as ours ; for they can reflect only 
the light which they receive from the sun ; and both light 
and heat decrease in the same ratio or proportion to the 
distances as gravity. Can you tell me now how much 
more light we enjoy than Saturn ? 

Caroline. The square of ten, is a hundred ; therefore 
Saturn has a hundred times less — or to answer your ques- 
tion exactly, we have a hundred times more light and heat 
than Saturn — this certainly does not increase my wish 
to become one of the poor wretches who inhabit that 
planet.f 

* This ring is set edgewise round it, and the distance of the ring 
from the planet is equal to the breadth of the ring. The sun shines 
for almost fifteen of our years together on the northern side of the 
ring ; then goes off, and shines as long on the southern side of it, 
so there is but one day and one night on each side of the ring, in 
the time of Saturn's whole revolution about the sun, which takes 
up almost thirty of our years. 

f The sun's light at Saturn is 1000 times as great as the light of 
the full moon is to us. 

362. What planet is next in order as to distance from the sun ? 

363. What is its distance from the sun ? 364. In what 

time does it revolve round that luminary ? 365. What is its 

diameter ? 366. How many moons has Saturn ?— 367. By 

what is this planet surrounded ? 368. What is said in the 

note of Saturn's ring f 369. How many moons has Herschel 

or the Georgium Sidus ? 370. How much more light and heat 

do we enjoy than Saturn ? 371. How much greater is the 

suns light at Saturn than the moons light at the tdrth ? 



96 ON THE PLANETS. 

Mrs, B. May not the inhabitants of Mercury, with 
equal plausibility, pity us, for the insupportable coldness 
of our situation, and those of Jupiter and Saturn for our 
intolerable heat ? The Almighty Power which created 
these planets, and placed them in their several orbits, has 
no doubt peopled them with beings whose bodies are 
adapted to the various temperatures and elements in 
which they are situated. If we judge from the analogy 
of our own earth, or from that of the great and universal 
beneficence of Providence, we must conclude this to be 
the case. 

Caroline, Are not comets also supposed to be planets ? 

Mrs. B. Yes, they are ; for by the re-appearance of 
some of them, at stated times, they are known to revolve 
round the sun, but in orbits so extremely eccentrick, that 
they disappear for a great number of years. If they are 
inhabited, it must be by a species of beings very different, 
not only from the inhabitants of this, but from those of any 
of the other planets, as they must experience the greatest 
vicissitudes of heat and cold ; one part of their orbit being 
so near the sun, that their heat, when there, is computed 
to be greater than that of red-hot iron ; in this part of its 
orbit, the comet emits a luminous vapour, called the tail, 
which it gradually loses as it recedes from the sun ; and 
the comet itself totally disappears from our sight, in the 
more distant parts of its orbit, which extends considerably 
beyond that of the furthest planet. 

The number of comets belonging to our system cannot 
be ascertained, as some of them ave whole centuries be- 
fore they make their re-appearance. The numbers that 
are known by their regular re-appearance is only three.* 

Emily. Pray, Mrs. B. what are the constellations ? 



* Above 500 comets have appeared since the commencement of 
the Christian era ; and accounts of many more are extant. 



372. What are the comets supposed to be ? -373. From 

what fact is it concluded that the comets are planets ? 374. 

Why must the inhabitants of comets, if they are inhabited, expe- 
rience great vicissitudes of heat and cold ? 375. When in that 

part of their orbit nearest the sun, what is their heat computed to 
he ? 376. How many comets are known by their regular re- 
appearance ? 377. How many different ones have been noticed 

since the commencement of the Christian era? 



ON THE PLANETS. 9? 

Mrs. B. They are the fixed stars, which the ancients, 
in order to recognise them, formed into groupes, and 
gave the names of the figures, which you find delineated 
on the celestial globe. In order to show their proper situ- 
ations in the heavens, they should be painted on the in- 
ternal surface of a hollow sphere, from the centre of which 
you should view them ; you would then behold them, as 
they appear to be situated in the heavens. The twelve con- 
stellations, called the signs of the zodiack, are those which 
are so situated, that the earth in its annual revolution passes 
directly between them and the sun. Their names are 
Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scor- 
pio, Sagittarius, Capricornus, Aquarius, Pisces ; the 
whole occupying a complete circle, or broad belt, in the 
heavens, called the zodiack. (plate VIII. fig. 1.) Hence, 
a right line drawn from the earth, and passing through 
the sun, would reach one of these constellations, and the 
sun is said to be in that constellation at which the line 
terminates : thus, when the earth is at A, the sun would 
appear to be in the constellation or sign Aries ; when the 
earth is at B, the sun would appear in Cancer ; when 
the earth was at C, the sun would be in Libra ; and when 
the earth was at D, the sun would be in Capricorn. This 
circle, in which the sun thus appears to move, and which 
passes through the middle of the zodiack, is called the 
ecliptick. 

Caroline. But many of the stars in these constella 
tions appear beyond the zodiack. 

Mrs. B. We have no means of ascertaining the dis- 
tance of the fixed stars. When, therefore, they are said 
to be in the zodiack, it is merely implied, that they are 
situated in that direction, and that they shine upon us 
through that portion of the heavens, which we call the 
zodiack.* 



* An easy distinction between a planet and a fixed star is this — 

373. What are the constellations ? -379. In what manner 

can we have an idea of their proper situations ? 380. What 

are the names of the twelve constellations ? 381 . What is meant 

when the sun is said to be in a particular constellation ? 382- 

How would you illustrate this by the figure ? — —383. What is the 
circle called in which the sun appears to move through the zo- 
diack ? 384. What is to be understood by the signs or con- 
stellations being in the zodiack ? 385. Row may a fixed star 

be easily distinguished from a planet f 

9 



98 ON THE PLANETS. 

Emily, But are not those large bright stars, which are 
called stars of the first magnitude, nearer to us, than 
those small ones which we can scarcely discern ? 

Mrs. B. It may be so ; or the difference of size and 
brilliancy of the stars may proceed from their difference 
of dimensions ; this is a point which astronomers are not 
enabled to determine. Considering them as suns, I see 
no reason why different suns should not vary in dimen- 
sions as well as the planets belonging to them.* 

Emily. What a wonderful and beautiful system this 
is, and how astonishing to think that every fixed star may 
probably be attended by a similar train of planets ! 

Caroline. You will accuse me of being very incredu- 
lous, but I cannot help still entertaining some doubts, and 
fearing that there is more beauty than truth in this system. 
It certainly may be so ; but there does not appear to 
me to be sufficient evidence to prove it. It seems so 
plain and obvious that the earth is motionless, and that 
the sun and stars revolve round it ; your solar system, 
you must allow, is directly in opposition to the evidence 
of our senses. 

Mrs. B. Our senses so often mislead us, that we 
should not place implicit reliance upon them. 

Caroline. On what then can we rely, for do we not 
receive all our ideas through the medium of our senses ? 

Mrs. B. It is true that they are our primary source of 
knowledge ; but the mind has the power of reflecting, 
judging, and deciding upon the ideas received by the or- 
gans of sense. This faculty, which we call reason, has 
frequently proved to us, that our senses are liable to err. 



the former shines with a steady light, but the latter is constantly 
twinkling. What it is whicli occasions this twinkling or scintilla- 
tion of a star, yet remains undecided. 

* To the bare eye the stars appear of some sensible magnitude, 
owing to the glare of light arising from the numberless reflections 
of the rays in coining to the eye ; this leads us to imagine that the 
stars are much larger than they would appear, if we saw them only 
by the few rays which come directly from them, so as to enter the 
eye, without being intermixed with others. 



386. On what is the different size and brilliancy of the fixed 

stars depending ? 387. What causes the fixed stars to appear 

to us larger than they should appear? 



ON THE PLANETS. 99 

If you have ever sailed on the water, with a very steady 
breeze, you must have seen the houses, trees, and every 
object move, while you were sailing. 

Caroline. I remember thinking so, when I was very 
young ; but I now know that their motion is only appa- 
rent. It is true that my reason, in this case, corrects the 
errour of my sight. 

Mrs. B. It teaches you that the apparent motion of 
the objects on shore, proceeds from your being yourself 
moving, and that you are not sensible of your own motion 
because you meet with no resistance. It is only when 
some obstacle impedes our motion, that we are conscious 
of moving ; and if you were to close your eyes when you 
were sailing on calm water, with a steady wind, you 
would not perceive that you moved, for you could not feel 
it, and you could see it only by observing the change of 
place of the objects on shore. So it is with the motion of 
the earth ; every thing on its surface, and the air that 
surrounds it, accompanies it in its revolution ; it meets 
with no resistance : therefore, like the crew of a vessel 
sailing with a fair wind, in a calm sea, we are insensible 
of our motion. 

Caroline. But Jhe principal reason why the crew of a 
vessel in a calm sea do not perceive their motion, is, be- 
cause they move exceedingly slowly ; while the earth, 
you say, revolves with great velocity. 

Mrs. B. It is not because they move slowly, but be- 
cause they move steadily, and meet with no irregular re- 
sistances, that the crew of a vessel do not perceive their 
motion ; for they would be equally insensible to it, with 
the strongest wind, provided it were steady, that they 
sailed with it, and that it did not agitate the water ; but 
this last condition, you know, is not possible, for the wind 
will always produce waves which offer more or less resist- 
ance to the vessel, and then the motion becomes sensible, 
because it is unequal. 

Caroline. But, granting this, the crew of a vessel have 
a proof of their motion, though insensible, which the in- 
habitants of the earth cannot have, — the apparent motion 
of the objects on shore. 

388. What familiar illustration is given to show why we do 

Dot perceive the motion of the earth in its revolutions ? 389. 

Why do we not perceive its motion ? 



100 ON THE PLANETS 

Mrs. B. Have we not a similar proof of the earth's 
motion, in the apparent motion of the sun and stars ? Ima- 
gine the earth to be sailing round its axis, and succes- 
sively passing by every star, which, like the objects on 
land, we suppose to be moving instead of ourselves. I 
have heard it observed by an aerial traveller in a balloon, 
tha*t the earth appears to ^ink beneath the balloon, in- 
stead of the balloon rising above the earth. 

It is a law which we discover throughout nature, and 
worthy of its great Author, that all its purposes are ac- 
complished by the most simple means ; and what reason 
have we to suppose this law infringed, in order that we 
may remain at rest, while the sun and stars move round 
us ; their regular motions, which are explained by the 
laws of attraction on the first supposition, would be un- 
intelligible on the last, and the order and harmony of the 
universe be destroyed. Think what an immense circuit 
the sun and stars would make daily, were their apparent 
motions real. We know many of them to be bodies 
more considerable than our earth ; for our eyes vainly 
endeavour to persuade us, that they are little brilliants 
sparkling in the heavens, while science teaches us that 
they are immense spheres, whose apparent dimensions 
are diminished by distance. Why then should these 
enormous globes daily traverse such a prodigious space, 
merely to prevent the necessity of our earth's revolving 
on its axis 1 

Caroline. I think I must now be convinced. But 
you will, I hope, allow me a little time to familiarize my- 
self to an idea so different from that which I have been 
accustomed to entertain. And pray, at what rate do we 
move ? 

Mrs. 13. The motion produced by the revolution of 
the earth on its axis, is about eleven miles a minute, to an 
inhabitant of London. 

Emily. But does not every part of the earth move 
with the same velocity 1 

390. In case the earth revolves every 24 hours, do not the sun 

and stars appear to us as if they revolved about the earth ? 391 

What law is mentioned that we discover throughout nature ? 

392. Why does this law make it more probable that the earth re 

volves than that the sun and stars do ? 393. How fast doe* 

a person move in the latitude of London, in consequence of the 
earth's motion upon its azii ? 



ON THE PLANETS. 101 

Mrs. B* A moment's reflection would convince you 
of the contrary ; a person at the equator must move 
quicker than one situated near the poles, since they both 
perform a revolution in 24 hours. 

Emily. True, the equator is furthest from the axis of 
motion. But in the earth's revolution round the sun, 
every part must move with equal velocity ? 

Mrs. B. Yes, about a thousand miles a minute. 

Caroline. How astonishing ! — and that it should be 
possible for us to be insensible of such a rapid motion. 
You would not tell me this sooner, Mrs. B., for fear of 
increasing my incredulity. 

Before the time of Newton, was not the earth supposed 
to be in the centre of the system, and the sun, moon, and 
stars to revolve round it 1 

Mrs. B. This was the system of Ptolemy in ancient 
times ; but as long ago as the beginning of the sixteenth 
century it was discarded, and the solar system, such as 
I have shown you, was established by the celebrated as- 
tronomer Copernicus, and is hence called the Copernican 
system. But the theory of gravitation, the source from 
which this beautiful and harmonious arrangement flows, 
we owe to the powerful genius of Newton, who lived at a 
much later period. 

Emily. It appears, indeed, far less difficult to trace 
by observation the motion of the planets, than to divine 
by what power they are impelled and guided. I wonder 
how the idea of gravitation could first have occurred to 
Sir Isaac Newton 1 

Mrs. B. It is said to have been occasioned by a cir- 
cumstance from which one should little have expected so 
grand a theory to have arisen. 

During the prevalence of the plague in the year 1665, 
Newton retired into the country to avoid the contagion : 
when sitting one day in his orchard he observed an apple 
fall from a tree, and was led to consider what could be 
the cause which brought it to the ground. 

394. How fast does the earth move in its revolution about the 
sun ? 395. What was the system of Ptolemy concerning as- 
tronomy ? 396. What is the present system of astronomy 

called ? — — 397. When was the Copernican system of astronomy 

adopted ? 398. What important discovery did Newton make 

touching the Copernican system ?- 399. What led Newton to 

make his discoveries ? 

9* 



102 ON THE EARTH. 

Caroline. If I dared to confess it, Mrs. B., I should 
say that such an inquiry indicated rather a deficiency 
than a superiority of intellect. I do not understand how 
any one can wonder at what is so natural and so common. 

Mrs. B. It is the mark of superiour genius to find 
matter for wonder, observation, and research, in circum- 
stances which, to the ordinary mind, appear trivial, be- 
cause they are common, and with which they are satis- 
fied, because they are natural, without reflecting that na- 
ture is our grand field of observation, that within it is con- 
tained our whole store of knowledge; in a word, that to 
study the works of nature, is to learn to appreciate and 
admire the wisdom of God. Thus, it was the simple cir- 
cumstance of the fall of an apple, which led to the discovery 
of the laws upon which the Copernican system is found- 
ed ; and whatever credit this system had obtained before, 
it now rests upon a basis from which it cannot be shaken. 

Emily. This was a most fortunate apple, and more 
worthy to be commemorated than all those that have been 
sung by the poets. The apple of discord for which the 
goddesses contended ; the golden apples by which Ata- 
lanta won the race ; nay, even the apple which William 
Tell shot from the head of his son, cannot be compared 
to this ! 



CONVERSATION VIII. 



ON THE EARTH. 



Of the Terrestrial Globe ; Of the Figure of the Earth ; 
Of the Pendulum; Of the Variation of the Seasons, 
and of the Length of Days and Nights ; Of the Causes 
of the Heat of Summer ; Of Solar, Sidereal, and Equal 
or Mean Time. 



MRS. B. 



As the earth is the planet in which we are the most 
particularly interested, it is my intention this morning, 
to explain to you the effects resulting from its annual and 

400 What does Mrs. Marret consider a mark of superiour genius ? 



ON THE EARTH. 103 

diurnal motions ; but for this purpose it will be necessa- 
ry to make you acquainted with the terrestrial globe : you 
have not either of you, I conclude, learnt the use of the 
globes ? # 

Caroline. No ; I once indeed learnt by heart the 
names of the lines marked on the globe, but as I was in- 
formed they were only imaginary divisions, they did not 
appear to me worthy of much attention, and w r ere soon 
forgotten. 

Mrs. B. You suppose, then, that astronomers had 
been at the trouble of inventing a number of lines to little 
purpose. It will be impossible for me to explain to you 
the particular effects of the earth's motion without your 
having acquired a knowledge of these lines : in plate 
VIII. fig. 2. you will find them all delineated ; and you 
must learn them perfectly if you wish to make any profi- 
ciency in astronomy. 

Caroline. I was taught them at so early an age that I 
could not understand their meaning ; and I have often 
heard you say that the only use of words was to convey 
ideas. 

Mrs. B. The names of these lines would have con- 
veyed ideas of the figures they were designed to express, 
though the use of these figures might at that time have been 
too difficult for you to understand. Childhood is the sea- 
son when impressions on the memory are most strongly and 
most easily made : it is the period at which a large stock of 
ideas should be treasured up, the application of which we 
may learn when the understanding is more developed. 
It is, I think, a very mistaken notion that children should 
be taught such things only, as they can perfectly under- 



* The earth is of a globular form. For, 1. The shadow of the 
earth projected on the moor, in an eclipse is always circular ; 
which appearance could only be produced by a spherical body. 
2. The convexity of the surface_of the sea is evident ; the mast of 
an approaching ship being seen before its hull. 3. The north pole 
becomes more elevated by travelling northward, in proportion to 
the space passed over. 4. Navigators have sailed round the earth, 
and by steering their course continually westward arrived, at 
length, at the place from whence they departed. 

401. How is it proved that the earth is globular f 1 02. 
What is necessary to be learnt before one can understand the of 
fects resulting from the earth's motions ? 



04 ON THE EARTH. 

stand. Had you been early made acquainted with the 
terms which relate to figure and motion, how much it 
would have facilitated your progress in natural philoso- 
phy ! I have been obliged to confine myself to the most 
common and familiar expressions, in explaining the laws 
of nature, though I am convinced that appropriate and 
scientifick terms would have conveyed more precise and 
accurate ideas ; but I was afraid of not being understood. 

Emily. You may depend upon our learning the names 
of these lines thoroughly, Mrs. B. ; but before we com- 
mit them to memory, will you have the goodness to ex- 
plain them to us 1 

Mrs. B. Most willingly. This globe, or sphere, 
represents the earth ; the line which passes through its 
centre, and on which it turns, is called its axis, and the 
two extremities of the axis A and B, are the poles, dis- 
tinguished by the names of the north and south pole. 
The circle C D, which divides the globe into two equal 
parts between the poles, is called the equator, or equi- 
noctial line ; that part of the globe to the north of the 
equator is the northern hemisphere ; that part to the 
south of the equator, the southern hemisphere. The 
small circle E F, which surrounds the north pole, is call- 
ed the arctick circle ; that G H, which surrounds the 
south pole, the antarctick circle. There are two inter- 
mediate circles between the polar circles and the equator ; 
that to the north, I K, called the tropick of Cancer ; that 
to the south, L M, called the tropick of Capricorn. 
Lastly, this circle, L K, which divides the globe into 
two equal parts, crossing the equator and extending 
northward as far as the tropick of Cancer, and southward 
as far as the tropick of Capricorn, is called the ecliptick. 
The delineation of the ecliptick on the terrestrial globe is 
not without danger of conveying false ideas ; for the 
ecliptick (as I have before said) is an imaginary circle in 
the heavens passing through the middle of the zodiack, and 
situated in the plane of the earth's orbit. 

403. What, in an artificial globe, represents the earth's axis? 
-404. What are the extremities of the axis called ? 405. 



What is the equator ? 40(i. What line in the figure represents 

the equator ? — What ones the Tropicks . J — What ones the Polar 

Circles ? — What one the Ecliptick ? 407. By what name are 

the two tropicks distinguished from each other ? 408. By 

what name are the polar circles distinguished from each otlic"' ? 
• -409. Where is the ecliptick situated ? 



ON THE EARTH. 105 

Caroline. I do not understand the meaning of the 
plane of the earth's orbit. 

Mrs. B. A plane, or plain, is an even level surface 
Let us suppose a smooth thin solid plane cutting the sun 
through the centre, extending out as far as the fixed 
stars, and terminating in a circle which passes through 
the middle of the zodiack ; in this plane the earth would 
move in its revolution round the sun ; it is therefore 
called the plane of the earth's orbit, and the circle in 
which this plane cuts the signs of the zodiack is the eclip- 
tick. Let the fig. 1. plate IX. represent such a plane, S 
the sun, E the earth with its orbit, and A BCD the 
ecliptick passing through the middle of the zodiack. 

Emily. If the ecliptick relates only to the heavens, 
why is it described upon the terrestrial globe 1 

Mrs. B. It is convenient for the demonstration of a 
variety of problems in the use of the globes ; and besides, 
the obliquity of this circle to the equator is rendered more 
conspicuous by its being described on the same globe ; 
and the obliquity of the ecliptick shows the inclination of 
the earth's axis to the plane of its orbit. But to return 
to fig. 2. plate VIII. 

The spaces between the several parallel circles on the 
terrestrial globe are called zones ; that which is compre* 
hended between the tropicks is distinguished by the name 
of the torrid zone ; the spaces which extend from the 
tropicks to the polar circles, the north and south tempe- 
rate zones ; and the spaces contained within the polar 
circles, the frigid zones. 

The several lines which, you observe, are drawn from 
one pole to the other, cutting the equator at right angles, 
are called meridians. When any one of these meridians 
is exactly opposite the sun it is mid-day, or twelve o'clock 
in the day, with all the places situated on that meridian ; 
and, with the places situated on the opposite meridian, it 
is consequently midnight. 

410. What is to be understood by the plane of the earth's orbit ? 
411. By what figure is it represented ? 412. If the eclip- 
tick relate only to the heavens, why is it described on the ter- 
restrial globe ? 413. What are called the zones ? 414. 

Where is the torrid zone ? 415. Where are the temperate 

zones ? 416. Where are the frigid zones ? 417. What are 

the meridian lines ? 418. When is it twelve o'clock at noon 

to all places under any particular meridian ? 419. To what 

places will it, at the same time, be midnight ? 



106 ON THE EARTH. 

Emily. To places situated equally distant from these 
two meridians, it must then be six o'clock 1 

Mrs. B. Yes ; if they are to the east of the sun's 
meridian it is six o'clock in the afternoon, because the 
sun will have previously passed over them ; if to the 
west, it is six o'clock in the morning, and the sun will 
be proceeding towards that meridian. 

Those circles which divide the globe into two equal 
parts, such as the equator and the ecliptick, are called 
greater circles ; to distinguish them from those which di- 
vide it into two unequal parts, as the tropicks and polar 
circles, which are called lesser circles. All circles are 
divided into 360 equal parts, called degrees, and degrees 
into 60 equal parts, called minutes. The diameter of a 
circle is a right line drawn across it, and passing through 
the centre ; for instance, the boundary of this sphere is 
a circle, and its axis the diameter of that circle ; the di- 
ameter is equal to a little less than one-third of the cir- 
cumference. Can you tell me nearly how many degrees 
it contains ? 

Caroline. It must be something less than one-third of 
360 degrees, or nearly 120 degrees. 

Mrs. B. Right ; now Emily you may tell me exactly 
how many degrees are contained in a meridian 1 

Emily. A meridian, reaching from one pole to the 
other, is half a circle, and must therefore contain 180 
degrees. 

Mrs. B. Very well ; and what number of degrees are 
there from the equator to the poles ? 

Caroline. The equator being equally distant from 
either pole, that distance must be half of a meridian, or 
a quarter of the circumference of a circle, and contain 90 
degrees. 

Mrs. B. Besides the usual division of circles into de- 
grees, the ecliptick is divided into 12 equal parts, 

420. To what places will it be six o'clock in the morning, and 

to what ones six in the evening ? 421. What circles are called 

greater circles ? 422. What ones are called lesser circles ? 

423. Into how many parts are all circles divided ? 424. 

How are degrees divided ?- 425. What is the diameter of a cir- 
cle ? 426. How many degrees does the diameter of a circle 

contain ? 427. How many degrees are there in a meridian 

reaching from one pole to the other ? 428. How many de- 
grees are there between the equator and the poles ? 429. How 

is the ecliptick divided ? 



ON THE EARTH. 107 

called signs, which bear the names of the constellations 
through which this circle passes in the heavens. The 
degrees measured on the meridians from north to south, 
or south to north, are called degrees of latitude ; those 
measured from east to west on the equator, the ecliptick, 
or any of the lesser circles, are called degrees of longi- 
tude ; hence these circles bear the name of longitudinal 
circles ; they are also called parallels of latitude. 

Emily, The degrees of longitude must then vary in 
length according to the dimensions of the circle on which 
they are reckoned ; those, for instance, at the polar cir- 
cles will be considerably smaller than those at the equa- 
tor ? 

Mrs. B. Certainly ; since the degrees of circles of 
different dimensions do not vary in number, they must 
necessarily vary in length. The degrees of latitude, you 
may observe, never vary in length ; for the meridians on 
which they are reckoned are all of the same dimensions. 

Emily. And of what length is a degree of latitude 1 

Mrs. B. Sixty geographical miles, which is equal to 
69^ English statute miles. • 

Emily. The degrees of longitude at the equator must 
then be of the same dimensions 1 

Mrs. B. They would, were the earth a perfect sphere ; 
but its form is not exactly spherical, being somewhat 
protuberant about the equator, and flattened towards the 
poles. This form is supposed to proceed from the superi- 
our action of the centrifugal power at the equator. 

Caroline. I thought I had understood the centrifugal 
force perfectly, but I do not comprehend its effect in this 
instance. 

Mrs. B. You know that the revolution of the earth 
on its axis must give every particle a tendency to fly off 
from the centre, that this tendency is stronger or weaker 
in proportion to the velocity with which the particle 
moves ; now a particle situated near one of the polar 
circles makes one rotation in the same space of time as a 



430. What is latitude ? 431. What is longitude ? 432. 

Are the degrees of longitude in different latitudes of the same 

length ? -433. What is the length of a degree of latitude ? 

434. What is the reason that a degree of longitude on the equa- 
tor is not the same as a degree of latitude ? 435. What occa- 
sions the protuberance of the earth at the equator r 



108 ON THE EARTH. 

particle at the equator ; the latter, therefore, having a 
much larger circle to describe, travels proportionally 
faster, consequently the centrifugal force is much stronger 
at the equator than at the polar circles : it gradually de- 
creases as you leave the equator and approach the poles, 
where, as there is no rotatory motion, it entirely ceases. 
Supposing, therefore, the earth to have been originally 
in a fluid state, the particles in the torrid zone would re- 
cede much further from the centre than those in the frigid 
zones ; thus the polar regions would become flattened, 
and those about the equator elevated. 

Caroline. I did not consider that the particles in the 
neighbourhood of the equator move with greater velocity 
than those about the poles ; this was the reason I could 
not understand you. 

Mrs. B. You must be careful to remember, that those 
parts of a body which are furthest from the centre of mo- 
tion must move with the greatest velocity : the axis of 
the earth is the centre of its diurnal motion, and the equa- 
torial regions the parts most distant from the axis. 

Caroline. My head then moves faster than my feet ; 
and upon the summit of a mountain we are carried round 
quicker than in a valley ? 

Mrs. B. Certainly, your head is more distant from 
the centre of motion, than your feet ; the mountain-top 
than the valley : and the more distant any part of a body 
is from the centre of motion, the larger is the circle it 
will describe, and the greater therefore must be its ve- 
locity. 

Emily. I have been reflecting that if the earth is not 
a perfect circle 

Mrs. B. A sphere you mean, my dear ; a circle is a 
round line, every part of which is equally distant from the 
centre ; a sphere or globe is a round body, the surface of 
which is every where equally distant from the centre. 

Emily. If then, the earth is not a perfect sphere, 
but prominent at the equator, and depressed at the poles, 
would not a body weigh heavier at the equator than at 
the poles 1 For the earth being thicker at the equator, 

436. In what manner can you account for this protuberance 
from centrifugal motion ? 437. Why does the head of a per- 
son move faster than his feet in the revolution of the earth upon 
|ti axis ? 438. What is a sphere or globe ? 



ON THE EARTH. 109 

the attraction of gravity perpendicularly downwards must 
be stronger. 

Mrs. B. Your reasoning has some plausibility, but I 
am sorry to be obliged to add that it is quite erroneous ; 
for the nearer any part of the surface of a body is to the 
centre of attraction, the more strongly it is attracted ; 
because the most considerable quantity of matter is about 
that centre. In regard to its effects, you might consider 
the power of gravity, as that of a magnet placed at the 
centre of attraction. 

Emily. But were you to penetrate deep into the 
earth, would gravity increase as you approached the 
centre 7 

Mrs. B. Certainly not ; 1 am referring only to any 
situation on the surface of the earth. Were you to pene 
trate into the interiour, the attraction of the parts above 
you would counteract that of the parts beneath you, and 
consequently diminish the power of gravity in proportion 
as you approached the centre ; and if you reached that 
point, being equally attracted by the parts all around 
you, gravity would cease, and you would be without 
weight. 

Emily. Bodies then should weigh less at the equator 
than at the poles, since they are more distant from the 
centre of gravity in the former than in the latter situation. 

Mrs. B. And this is really the case ; but the diffe- 
rence of weight would be scarcely sensible, were it not 
augmented by another circumstance. 

Caroline. And what is this singular circumstance 
which seems to disturb the laws of nature ? 

Mrs. B. One that you are well acquainted with, as 
conducing more to the preservation than the destruction 
of order, — the centrifugal force. This we have just ob- 
served to be stronger at the equator ; and as it tends to 
drive bodies from the centre, it is necessarily opposed to, 
and must lessen the power of gravity, which attracts 
them towards the centre. We accordingly find that bo- 



439. Will any body weigh the same at the equator as at the 

poles ? 440. Were one to penetrate deep into the earth, 

would the force of gravity increase ? 441. Why not ? 442, 

Where will bodies weigh most,- at the equator or poles ? =443, 

What besides the protuberance at the equator causes bodies to 
weigh less there than at the poles ? 
10 



110 ON THE EARTH. 

dies weigh lightest at the equator, where the centrifugal 
force is greatest ; and heaviest at the poles, where this 
power is least.* 

Caroline. Has the experiment been made in these 
different situations 1 

Mrs. B. Louis XIV., of France, sent philosophers 
both to the equator and to Lapland for this purpose ; the 
severity of the climate, and obstruction of the ice, have 
hitherto rendered every attempt to reach the pole abor- 
tive ; but the difference of gravity at the equator and in 
Lapland is very perceptible. 

Caroline. Yet I do not comprehend, how the diffe- 
rence of weight could be ascertained ; for if the body un- 
der trial decreased in weight, the weight which was op- 
posed to it in the opposite scale must have diminished in 
the same proportion. For instance, if a pound of sugar 
did not weigh so heavy at the equator as at the poles, the 
leaden pound which served to weigh it, would not be so 
heavy either : therefore they would still balance each 
other, and the different force of gravity could not be as- 
certained by this means. 

Mrs. B. Your observation is perfectly just : the diffe- 
rence of gravity of bodies situated at the poles and at 
the equator cannot be ascertained by weighing them ; a 
pendulum was therefore used for that purpose. 

Caroline. What, the pendulum of a clock 1 how could 
that answer the purpose ? 

Mrs. B. A pendulum consists of a line, or rod, to 
one end of which a weight is attached, and it is suspend- 
ed by the other to a fixed point, about which it is made 



* If the diurnal motion of the earth round its axis were about 
17 times faster than it is, the centrifugal force would, at the equa- 
tor, be equal to the power of gravity, and all bodies there would 
entirely lose weight. But if the earth revolved still quicker than 
this, they would all fly off. 



444. How mv ch faster must the earth move than it now does to 
have the centrifugal force balance that of gravity, and thereby 
cause bodies entirely to lose their weight ? 445. Has an at- 
tempt ever been made to ascertain whether bodies will weigh hea- 
vier at the poles than at the equator ? 446. By whom was it 

made ? 447. Could the experiment be made by the common 

ecales ? 448. Why not ? 449. What instrument was used in 

the experiment ? 450. How would you describe a pendulum ' 



ON THE EARTH. Ill 

to vibrate. Without being put in motion, a pendulum, 
like a plumb line, hangs perpendicular to the general sur- 
face of the earth, by which it is attracted ; but, if you 
raise a pendulum, gravity will bring it back to its perpen- 
dicular position. It will, however, not remain stationary 
there, for the velocity it has received during its descent 
will impel it onwards, and it will rise on the opposite side 
to an equal height ; from thence it is brought back by 
gravity, and again driven by the impulse of its velocity. 

Caroline. If so, the motion of a pendulum would be 
perpetual, and I thought you said that there was no per- 
petual motion on the earth. 

Mrs. B. The motion of a pendulum is opposed by the 
resistance of the air in which it vibrates, and by the fric- 
tion of the part by which it is suspended ; were it possible 
to lemove t^ese obstacles, the motion of a pendulum 
would be perpetual, and its vibrations perfectly regular ; 
being of equal distances, and performed in equal times.* 

Emily. That is the natural result of the uniformity of 
the power which produces these vibrations, for the force 
of gravity being always the same, the velocity of the pen- 
dulum must consequently be uniform. 

Caroline. No, Emily, you are mistaken ; the cause is 
not always uniform, and therefore the effect will not be so 
either. I have discovered it, Mrs. B. : since the force of 
gravity is less at the equator than at the poles, the vibra- 
tions of the pendulum will be slower at the equator than 
at the poles. 



* The vibrations of pendulums are subject to many irregularities 
for which no effectual remedy has yet been devised. These are 
owing partly to the variable density and temperature of the air, 
partly to the rigidity and friction of the rod by which they are sus- 
pended, and principally to the dilatation and contraction of the ma- 
terials, of which they are formed. The metalline rods of pendu- 
lums are expanded by heat, and contracted by cold ; therefore 
clocks will go faster in winter, and slower in summer. The com- 
mon remedy for this inconvenience is the raising or lowering t!;e 
bob of the pendulum, by means of a screw, as occasion may re- 
quire. 

451. What causes the vibrations of a pendulum ? 452. Why 

are not its vibrations perpetual ? 453. To what is the irregu 

larity in the vibrations of pendulums owing ? 454. Why xoill 

clocks go faster in winter than in summer ? 455. Where do 

pendulums of the same length vibrate fastest ? 



112 ON THE EARTH. 

Mrs. B. You are perfectly right, Caroline ; it was 
by this means that the difference of gravity was discover- 
ed, and the true figure of the earth ascertained. 

Emily. But how do they contrive to regulate their 
time in the equatorial and polar regions 1 for, since in 
this part of the earth the pendulum of a clock vibrates 
exactly once in a second, if it vibrates faster at the poles 
and slower at the equator, the inhabitants must regulate 
their clocks in a different manner from ours. 

Mrs. B. The only alteration required is to lengthen 
the pendulum in one case, and to shorten it in the other ; 
for the velocity of the vibrations of a pendulum depends 
on its length ; and when it is said, that a pendulum vi- 
brates quicker at the pole than at the equator, it is sup- 
posing it to be of the same length. A pendulum which 
vibrates a second in this latitude is 36^ inches long. In 
order to vibrate at the equator in the same space of time, 
it must be lengthened by the addition of a few lines ; 
and at the poles, it must be proportionally shortened.* 

I shall now, I think, be able to explain to you the va- 
riation of the seasons, and the difference of the length of 
the days and nights in those seasons ; both effects result- 
ing from the same cause. 

In moving round the sun, the axis of the earth is not 
perpendicular to the plane of its orbit. Supposing this 
round table to represent the plane of the earth's orbit, and 
this little globe, which has a wire passing through it, re- 
presenting the axis and poles, we shall call the earth ; in 
moving round the table, the wire is not perpendicular to 
it, but oblique. 



* What is here stated concerning the length of pendulums as 
connected with the force of gravity is at complete variance with 
fact. The force of gravitation is greater, it is well known, at the 
poles than at the equator ; and since the vibration of pendulums 
is occasioned by gravity, the lengths of pendulums vibrating in the 
same time must evidently be proportioned to the gravities at the 
places where they vibrate. Accordingly, it is found, by observa- 
tion, in order to vibrate, at the equator, in the same space, the 
pendulum must not be lengthened, as above stated, but shortened ; 
and at the poles, it must not be shortened, but proportionally 
lengthened. 



450. How do the pendulums used at the equator and at the polar 
regions compare in length in order to vibrate in the same time ? 



ON THE EARTH. 1 13 

Emily. Yes, I understand the earth does not go round 
the sun in an upright position, its axis is slanting or ob- 
lique. 

Mrs. B. All the lines, which you learnt in your last 
lesson, are delineated on this little globe ; you must con= 
sider the ecliptick as representing the plane of the earth s 
orbit ; and the equator which crosses the ecliptick in two 
places, shows the degree of obliquity of the axis of the 
earth in that orbit, which is exactly 23^ degrees. The 
points in which the ecliptick intersects the equator are call- 
ed nodes. 

But I believe I shall make this clear to you by revolv- 
ing the little globe round a candle, which shall represent 
the sun. (Plate IX. fig. 2.) 

As I now hold it, at A, you see it in the situation in 
which it is in the midst of summer, or what is called the 
summer solstice, which is on the 21st of June. 

Emily. You hold the wire awry, I suppose, in order 
to show that the axis of the earth is not upright 1 

Mrs. B. Yes ; in summer, the north pole is inclined 
towards the sun. In this season, therefore, the northern 
hemisphere enjoys much more of his rays than the south- 
ern. The sun, you see, now shines over the whole of the 
north frigid zone, and notwithstanding the earth's diur- 
nal revolution, which I imitate by twirling the ball on 
the wire, it will continue to shine upon it as long as it 
remains in this situation, whilst the south frigid zone is 
at the same time completely in obscurity. 

Caroline. That is very strange : I never before heard 
that there was constant day or night in any part of Hie 
world ! How much happier the inhabitants of the north 
frigid zone must be than those of the southern ; the first 
enjoy uninterrupted day, while the last are involved in 
perpetual darkness. 

Mrs, B. You judge with too much precipitation ; ex- 
amine a little further, and you will find, that the two 
frigid zones share an equal fate. 

457. What causes the variation of the seasons and the diffe- 
rence of the length of the days and nights ? 453. How much is 

the axis of the earth inclined to the plane of its orbit ? 

459. What are the points called where the eciiptick intersects the 

equator ? 460. When does the summer solstice take place ? 

461. By which figure is the change of seasons illustrated ? 

46*2. When is the north pole inclined towards the sun ? 

463. What is the situation of the south pole when the north pole 
is inclined to the sun ? 

10* 



114 ON THE EARTH. 

We shall now make the earth set off from its position 
in the summer solstice, and carry it round the sun ; ob- 
serve that the pole is always inclined in the same direc- 
tion, and points to the same spot in the heavens. There 
is a fixed star situated near that spot, which is hence 
called the North Polar star. Now let us stop the earth 
at B, and examine it in its present situation ; it has gone 
through one quarter of its orbit, and is arrived at that 
point at which the ecliptick cuts or crosses the equator, 
and which is called the autumnal equinox. 

Emily. That is then one of the nodes. 

The sun now shines from one pole to the other, just as 
it would constantly do, if the axis of the earth were per- 
pendicular to its orbit. 

Mrs. B. Because the inclination of the axis is now 
neither towards the sun nor in the contrary direction ; at 
this period of the year, therefore, the days and nights are 
equal in every part of the earth. But the next step she 
lakes in her orbit, you see, involves the north pole in dark- 
ness, whilst it illumines that of the south ; this change 
was gradually preparing as I moved the earth from sum- 
mer to autumn ; the arctick circle, which was at first en- 
tirely illumined, began to have short nights, which in- 
creased as the earth approached the autumnal equinox; 
and the instant it passed that point, the long night of the 
north pole commences, and the south pole begins to enjoy 
the light of the sun. We shall now make the earth pro- 
ceed in its orbit, and you may observe that as it advances, 
the days shorten, and the nights lengthen, throughout the 
northern hemisphere, until it arrives at the winter solstice, 
on the 21st of December, when the north frigid zone is 
entirely in darkness, and the southern has uninterrupted 
day-light. 

Caroline. Then after all, the sun, which I thought so 
partial, confers his favours equally on all. 

Mrs. B. You mistake : the inhabitants of the torrid 
zone have much more heat than we have, as the sun's 
rays fall perpendicularly on them, while they shine ob- 

464. To what part of the heavens does the north pole always 

point ? 4G5. What part of the figure represents the earth at 

the autumnal equinox ? 466. How does the sun shine upon 

the earth at this season of the year ? 467. When is the winter 

solstice ? 468. Why is the heat of the sun greater at tho 

equator than at n distance from it ? 



ON THE EARTH. ] 15 

liquely on the rest of the world, and almost horizontally 
on the poles ; for during their long day of six months, the 
sun moves round their horizon without either rising or 
setting ; the only observable difference is, that it is more 
elevated by a few degrees at mid-day, than at mid-night. 

Emily. To a person placed in the temperate zone, in 

Jhe situation in which we are in England, the sun will 

shine neither so obliquely as it does on the poles, nor so 

vertically as at the equator ; but its rays will fall upon 

him more obliquely in autumn and winter, than in summer. 

Caroline. And therefore, the inhabitants of the tem- 
perate zones will not have merely one day and one night 
in the year as happens at the poles, nor will they have 
equal days and equal nights as at the equator ; but their 
days and nights will vary in length, at different times of 
the year, according as their respective poles incline to- 
wards or from the sun, and the difference will be greater 
in proportion to their distance from the equator. 

Mrs. B. We shall now follow the earth through the 
other half of her orbit, and you will observe, that now ex- 
actly the same effect takes place in the southern hemi- 
sphere, as what we have just remarked in the northern. 
Day commences at the south pole when night sets in at 
the north pole ; and in every other part of the southern 
hemisphere the days are longer than the nights, while, on 
the contrary, our nights are longer than our days. When 
the earth arrives at the vernal equinox, D, where the 
ecliptick again cuts the equator, on the 25th of March, she 
is situated with respect to the sun, exactly in the same 
position, as in the autumnal equinox ; and the only diffe- 
rence with respect to the earth, is, that it is now autumn 
in the southern hemisphere, whilst it is spring with us. 

Caroline. Then the days and nights are again every 
where equal ? 

Mrs. B. Yes, for the half of the globe which is en- 
lightened, extends exactly from one pole to the other, 
the day breaks to the north pole, and the sun sets to the 
south pole ; but in every other part of the globe, the day 
and night is of twelve hours' length, hence the word equi- 

469. In what direction do the rays of the sun fall upon the 

polar regions of the earth ? 470. When does day commence 

at the south pole ? 471. When does the earth arrive at the 

vernal equinox ? 472. What part of the figure exhibits the 

earth at the vernal equinox ? 



1 16 ON THE EARTH. 

nox, which is derived from the Latin, meaning equal 
night. 

As the earth proceeds towards summer, the days length- 
en in the northern hemisphere, and shorten in the south- 
ern, till the earth reaches the summer solstice, when the 
north frigid zone is entirely illumined, and the southern 
is in complete darkness ; and we have now brought the earth 
again to the spot from whence we first accompanied her. 

Emily. This is, indeed, a most satisfactory explana- 
tion of the seasons ; and the more I learn, the more I ad- 
mire the simplicity of means by which such wonderful 
effects are produced. 

Mrs. B. I know not which is most worthy of our 
admiration, the cause or the effect of the earth's revolu- 
tion round the sun. The mind can find no object of 
contemplation, more sublime, than the course of this mag- 
nificent globe, impelled by the combined powers of pro- 
jection and attraction to roll in one invariable course 
around the source of Tight and heat : and what can be 
more delightful than the beneficent effects of this vivify- 
ing power on its attendant planet ! It is at once the grand 
principle which animates and fecundates nature. 

Emily. There is one circumstance in which this little 
ivory globe appears to me to differ from the earth ; it is 
not quite dark on that side of it, which is turned from the 
candle, as is the case with the earth when neither moon 
nor stars are visible. 

Mrs. B. This is owing to the light of the candle 
being reflected by the walls of the room on every part of the 
globe, consequently that side of the globe on which the 
candle does not directly shine, is not in total darkness. 
Now the skies have no walls to reflect the sun's light on 
that side of our earth which is in darkness. 

Caroline. I beg your pardon, Mrs. B. I think that 
the moon and stars answer the purpose of walls in reflect- 
ing the sun's light to us in the night. 

Mrs. B. Very well, Caroline; that is to say, the 
moon and planets ; for the fixed stars, you know, shine 
by their own light. 

Emily. You say that the superiour heat of the equa- 
torial parts of the earth arises from the rays falling perpen- 
dicularly on those regions, whilst they fall obliquely on 
these more northern regions ; now I do not understand 

472. Why are the points where the ecliptick cuts or crosses the 
equator called equinoxes ? 



ON THE EARTH. 117 

vvhy perpendicular rays should afford more heat than ob- 
lique rays. 

Caroline. You need only hold your hand perpendicu- 
larly over the candle, and then hold it sideways obliquely, 
to be sensible of the difference. 

Emily. I do not doubt the fact, but I wish to have it 
explained. * 

Mrs. B. You are quite right ; if Caroline had not 
been satisfied with ascertaining the fact, without under- 
standing it, she would not have brought forward the can- 
dle as an illustration ; the reason why you feel so much 
more heat if you hold your hand perpendicularly over the 
candle, than if you hold it sideways, is because a steam 
of heated vapour constantly ascends from the candle or 
any other burning body, which being lighter than the air 
of the room, does not spread laterally but rises perpendi- 
cularly, and this led you to suppose that the rays were hot- 
ter in the latter direction. Had you reflected, you would 
have discovered that rays issuing from the candle side- 
ways, are no less perpendicular to your hand when held 
opposite to them, than the rays which ascend when your 
hand is held over them. 

The reason why the sun's rays afford less heat when 
in an oblique direction than when perpendicular, is be- 
cause fewer of them fall upon an equal portion of the 
earth ; this will be understood better by referring to plate 
X. fig. 1, which represents two equal portions of the sun's 
rays, shining upon different parts of the earth. Here it is 
evident that the same quantity of rays fall on the space A 
B as fall on the space B C ; and as A B is less than B C, 
the heat and light will be much stronger in the former than 
in the latter ; A B, you see, represents the equatorial re- 
gions, where the sun shines perpendicularly ; and B C, the 
temperate and frozen climates, where his rays fall more 
obliquely.* 

Emily. This accounts not only for the greater heat of 
the equatorial regions, but for the greater heat of summer ; 

* It is well known, that the south side of a hill, in our hemisphere, 
is peculiarly warm ; and the north side, peculiarly cold. This 
is owing to the d ifferent degrees of obliquity, with which the rays 

473. Why is the heat of perpendicular rays more intense than 

that of oblique ones ? 474. By which figure is this illustrated ? 

475. How will you explain Fig. 1, plate X. as illustrating 

this subject ? 476. Why is the south side of a kill warmer than 

the north side of it ? 



1 18 ON THE EARTH. 

as the sun shines less obliquely in summer than in 
winter. 

Mrs. B. This you will see exemplified in fig. 2, in 
which the earth is represented, as it is situated on the 21st 
June, and England receives less oblique, and consequently 
a greater number of rays, than at any other season ; 
and figure 3 shows the situation of England on the 21st 
December, when the rays of the sun fall most obliquely 
upon her. But there is also another reason why oblique 
rays give less heat, than perpendicular rays ; which is, 
that they have a greater portion of the atmosphere to tra- 
verse ; and though it is true that the atmosphere is itself 
a transparent body, freely admitting the passage of the 
sun's rays, yet it is always loaded more or less with dense 
and foggy vapour, which the rays of the sun cannot easily 
penetrate ; therefore the greater the quantity of atmo- 
sphere the sun's rays have to pass through in their way 
to the earth, the less heat they will retain when they 
reach it. This will be better understood by referring to 
figure 4. The dotted line round the earth, describes the 
extent of the atmosphere, and the lines which proceed 
from the sun to the earth, the passage of two equal por- 
tions of the sun's rays to the equatorial and polar regions ; 
the latter, you see, from its greater obliquity passes through 
a greater extent of atmosphere. 

Caroline. And this, no doubt, is the reason why the 
sun in the morning and the evening gives so much less 
heat, than at mid-day. 

Mrs. B. The diminution of heat, morning and even- 
ing, is certainly owing to the greater obliquity of the 
sun's rays ; and as such they are affected by both the 
causes, which I have just explained to you ; the difficul- 
ty of passing through a foggy atmosphere is perhaps more 
particularly applicable to them, as mists and vapours are 
very prevalent about the time of sunrise and sunset. 

of the sun strike the different sides of a hill. And a south-western 
is warmer than a south exposure, because it receives the sun : s rays 
in the warmest part of the day. 

477. Why is a south-western exposure to the sun warmer than 
a south exposure ? 478. What is to be illustrated by Figures 

2 &, 3 of plate X. ? 479. What is another reason why oblique 

rays give less heat than perpendicular ones ? 480. By which 

figure i9 the effect that the atmosphere has on the heat of the sun's 

ravs illustrated ? 481. Why doe9 the sun give more heat at 

mid-day, than in the morning and evening ? 



ON THE EARTH. 119 

But the diminished obliquity of the sun's rays is not the 
sole cause of the heat of summer ; the length of the days 
greatly conduces to it ; for the longer the sun is above the 
horizon, the more heat he will communicate to the earth. 

Caroline. Both the longest days, and the most perpen- 
dicular rays, are on the 21st of June ; and yet the great- 
est heat prevails in July and August. 

Mrs. B. Those parts of the earth which are once heat- 
ed, retain the heat for some length of time, and the addi- 
tional heat they receive, occasions an elevation of tem- 
perature, although the days begin to shorten, and the 
sun's rays fall more obliquely. For the same reason, we 
have generally more heat at three o'clock in the afternoon, 
than at twelve when the sun is on the meridian.* 



* There are also other causes which have an effect on tempera- 
ture. When the sun's rays strike upon the land, they are stop- 
ped and accumulated at the surface. They are then reflected 
into the air and to surrounding objects ; so that the reflected 
heat is often greater than tl e direct heat of the sun. Hence, the 
heat in valleys where the rf ys are reflected by the hills and moun- 
tains, is sometimes very gr^at. In an elevated valley in Switzer- 
land, the heat is so much" increased by reflection, that in the cen- 
tre there is a spot of perpetual verdure, in the midst of perpetual 
snows and glaciers; and there are plains on the Himmaleh moun- 
tains 15,000 feet above the level of the sea, which produce fine 
pasturage ; and, at the height of 11,000 feet, which is above the 
region of pe~etual snows on the Andes, in the same latitude, bar- 
ley and buckwheat flourish. But, unless heat is thus increased, it 
is reckoned as continually diminishing as we ascend above the 
level of the sea, especially on lofty mountains, where it is reflected 
into the dry, clear air around them, and is carried off by the winds 
which sweep over them, without any opportunity for accumula- 
tion. Thus an elevation of 500 yards produces the same effect as 
a distance of 5,000 miles from the equator. At the height of 
6,000 or 8,000 feet under the tropicks, we find the same climate as 
in latitude 49° in France. At 13,000 feet we find the frosts of 
the frigid zone ; and at 15,730 feet, the mountains, based upon 
the most scorching plains, are capped with perpetual snow. 

482. What, besides the direction of the sun's ravs, effects 

the temperature of the places where they fall ? 483* Why is 

it warmer in July and August than in June, when the days are 
longest ; and at 2 and 3 P. M. than at noon ? — —484. Why is the 

degree of heat increased in valleys ? 485. What fact is stated 

relating to this subject concerning a valley in Switzerland ? 

486. What facts are stated concerning the -plains of Himmaleh f 

487. How is temperature effected in ascending above the 

level of the sea ? -488. In what ratio, compared wiih the 

degrees of latitude, does heat diminish in rising above the level 
of the sea f 



120 ON THE EARTH. 

Emily. And pray, have the other planets the same vi 
cissitudes of seasons as the earth 1 

Mrs. B. Some of them more, some less, according 
as their axes deviate more or less from the perpendicular 
to the plane of their orbits. The axis of Jupiter is near- 
ly perpendicular to the plane of his orbit ; the axis of 
Mars and of Saturn are each inclined at angles of about 
sixty degrees ; whilst the axis of Venus is believed to be 
elevated only fifteen or twenty degrees above her orbit ; 
the vicissitudes of her seasons must therefore be conside- 
rably greater than ours. For further particulars respect- 
ing the planets, I shall refer you to Bonnycastle's Intro- 
duction to Astronomy. 



When the rays of the sun strike upon ttue water, they will pene- 
trate GOO or 700 feet, if there is that depth ; and the heat will be 
diffused through the mass, remaining, till carried off by evapo- 
ration. Consequently, in hot climates, the body of the ocean is 
much cooler than the land ; and in cold ones, it is warmer. Thus 
two countries which abound with rivers, lakes, and marshes, are 
also less subject to the extremes of heat and cold, than those which 
are dry. 

In addition to the direct effects of the sun, the different parts of 
the earth exert a continual influence on each other. The deserts 
of Arabia and Africa are like immense furnaces, in increasing the 
heat of all the regions on the Mediterranean sea, in the south of 
Europe and west of Asia. On the other hand, Siberia and the 
northern portions of North America have their cold increased by po- 
lar winds, which are not interrupted by mountains, while Europe 
is much protected from them by its mountains. 

The following may be considered a rule for determining the 
effect produced on temperature by winds. When the prevailing 
winds to which a country is exposed, come from polar or elevated 
regions, the cold is greater than the latitude would make it ; when 
they come from warmer regions, and especially from deserts, they 
increase the heat ; and when they come from the ocean, or large 
bodies of water, they diminish both heat and cold, according to 
the climate, rendering the temperature more uniform through the 
year. 

480. What facts arc mentioned relating to the rays of the sun 

falling upon water, as effecting temperature ? 490. What ones 

are mentioned relating to the influence which different portions of 
the earth exercise upon each other, as effecting temperature ? 
491. What is the rule named for determining the effects produced 

by the winds on temperature f 492. Have the other planets 

the same vicissitudes of seasons, that the earth has ? 493. 

Which planet has its axis nearly perpendicular to the plane of its 

orbit, ? 191. How are the axes of Mars and Saturn ? 495. 

How is the axis of Venus ; 



ON THE EARTH. 121 

I have but one more observation to make to you rela- 
tive to the earth's motion, which is, that although we 
have but 365 days and nights in the year, she performs 
366 complete revolutions on her axis during that time. 

Caroline. How is that possible ? for every complete 
revolution must bring the same place back to the sun. 
It is now just twelve o'clock, the sun is, therefore, on 
our meridian ; in twenty-four hours will it not be return- 
ed to our meridian again 1 and will not the earth have 
made a complete rotation on its axis ? 

Mrs. B. If the earth had no progressive motion in its 
orbit whilst it revolves on its axis, this would be the 
case ; but as it advances almost a degree westward in 
its orbit, in the same time that it completes a revolution 
eastward on its axis, it must revolve nearly one degree 
more in order to bring the same meridian back to the sun. 

Caroline. Oh, yes ! it will require as much more of a 
second revolution to bring the same meridian back to the 
sun, as is equal to the space the earth has advanced in 
her orbit, that is, nearly a degree ; this difference is, 
however, very little. 

Mrs. B. These small daily portions of rotation are 
each equal to the three hundred and sixty-fifth part of a 
circle, which at the end of the year amounts to one com- 
plete rotation. 

Emily. That is extremely curious. If the earth, 
then, had no other than its diurnal motion, we should 
have 366 days in the year. 

Mrs. B. We should have 366 days in the same period 
of time that we now have 365 ; but if we did not revolve 
round the sun, we should have no natural means of com- 
puting years. 

You will be surprised to hear, that if time is calculated 
by the stars instead of the sun, the irregularity which we 
have just noticed does not occur, and that one complete 
rotation of the earth on its axis, brings the same meridian 
back to any fixed star. 

Emily. That seems quite unaccountable ; for the 



496. To what is it owing that the earth performs 366 revolu- 
tions in a year that has but 365 days and nights ? 497. Under 

what circumstances should we have 366 days in the same period 
of time that we now have 365 ? — -498. In what way might tims 
be calculated so as to avoid this irregularity I 



122 ON THE EARTH. 

earth advances in her orbit with regard to the fixed stars 
the same as with regard to the sun. 

Mrs. B. True, but then the distance of the fixed stars 
is so immense, that our solar system is in comparison to 
it but a spot, and the whole extent of the earth's orbit but 
a point ; therefore, whether the earth remained stationary, 
or whether it revolved in its orbit during its rotation on 
its axis, no sensible difference would be produced with 
regard to the fixed stars. One complete revolution brings 
the same meridian back to the same fixed star ; hence the 
fixed stars appear to go round the earth in a shorter time 
than the sun by three minutes, fifty-six seconds of time. 

Caroline. These three minutes, fifty-six seconds is the 
time which the earth takes to perform the additional three 
hundred and sixty-fifth part of the circle, in order to bring 
the same meridian back to the sun. 

Mrs. B. Precisely. Hence the stars gain every day 
three minutes, fifty-six seconds on the sun, which makes 
them rise that portion of time earlier every day. 

When time is calculated by the stars it is called sidereal 
time, when by the sun, solar or apparent time.* 

Caroline. Then a sidereal day is three minutes, fifty- 
six seconds shorter than a solar day of twenty-four hours. 

Mrs. B. I must also explain to you what is meant by a 
sidereal year. 

The common year, called the solar or tropical year, 
containing 365 days, five hours, forty eight minutes, and 
fifty-two seconds, is measured from the time the sun sets 
out from one of the equinoxes, or solstices, till it returns 
to the same again ; but this year is completed before the 
earth has finished one entire revolution in its orbit. 



* If one clock should be so well regulated as to show the time 
to bo XII at noon this day, and on the 365th day afterward ; and 
another clock should be so well regulated as to show the time to 
be XII every day or night when any given star is on the meridian, 
the latter clock would gain three minutes, fifty-five seconds, and 
fifty-four sixtieth parts of a second upon the former in each revolu- 
tion of the same star to the meridian. 



499. Why do the fixed stars appear to revolve round the earth 

quicker than the sun ? 500. How much quicker than the sun 

do the fixed stars appear to go round the earth? 501. When 

is time called sidereal, and when solar or apparent time ? 502. 

What illustration is given of this in the note ? 503. What is 

the common or solar year ? 



ON THE EARTH, 123 

Emily. I thought that the earth performed one com- 
plete revolution in its orbit every year ; what is the rea- 
son of this variation 1 

Mrs. B. It is owing to the spheroidal figure of the 
earth. The elevation about the equator produces much 
the same effect as if a similar mass of matter, collected in 
the form of a moon, revolved round the equator. When 
this moon acted on the earth in conjunction with or in op- 
position to the sun, variations in the earth's motion would 
be occasioned, and these variations produce what is called 
the precession of the equinoxes. 

Emily. What does that mean ? I thought the equi- 
noctial points, or nodes, were fixed points in the heavens, 
in which the equator cuts the ecliptick. 

Mrs. B. These points are not quite fixed, but have an 
apparently retrograde motion, that is to say, instead of 
being every revolution in the same place, they move back- 
wards. Thus if the vernal equinox is at A, (fig. 1, plate 
XI.) the autumnal one will be at B instead of at C, and 
the following vernal equinox at D instead of at A, as 
would be the case if the equinoxes were stationary at op- 
posite points of the earth's orbit. 

Caroline. So that when the earth moves from one equi- 
nox to the other, though it takes half a year to perform 
the journey, it has not travelled through half its orbit. 

Mrs. B. And, consequently, when it returns again to 
the first equinox, it has not completed the whole of its 
orbit. In order to ascertain when the earth has perform- 
ed an entire revolution in its orbit, we must observe when 
the sun returns in conjunction with any fixed star ; and 
this is called a sidereal year. Supposing a fixed star si- 
tuated at E, (fig. 1, plate XI.) the sun would not appear in 
conjunction with it till the earth had returned to A, when 
it would have completed its orbit. 

Emily. And how much longer is the sidereal than the 
solar year ? 

504. What is the reason that the solar year is completed be- 
fore the earth has made one entire revolution in its orbit ? 

505. What is called the precession of the equinoxes, and how is 

it produced ? -506. How would you explain the precession of 

the equinoxes by the figure ? 507. How can it be ascertained 

when the earth has performed one entire revolution in its orbit ? 

508. What is a sidereal year ? 509. How much longer 

is the sidereal than the solar year ? 



124 ON THE MOON. 

Mrs. B. Only twenty minutes ; so that the variation 
of the equinoctial points is very inconsiderable. I have 
given them a greater extent in the figure in order to ren- 
der them sensible. 

Is regard to time, I must further add, that the earth's 
diurnal motion on an inclined axis, together with its an- 
nual revolution in an elliptick orbit, occasions so much 
complication in its motion, as to produce many irregula- 
rities ; therefore, true equal time cannot be measured by 
the sun. A clock, which was always perfectly correct, 
would in some parts of the year be before the sun, and in 
other parts after it. There are but four periods in which 
the sun and a perfect clock would agree, which is the 
loth of April, the 16th of June, the 23d of August, and 
the 24th of December. 

Emily. And is there any considerable difference be- 
tween solar time and true time ? 

Mrs. B. The greatest difference amounts to between 
fifteen and sixteen minutes. Tables of equation are con- 
structed for the purpose of pointing out and correcting 
these differences between solar time and equal or mean 
time, which is the denomination given by astronomers to 
true time. 

510. What are the periods, when the sun and a perfect clock 

agree ? 511. What is the greatest difference between solar 

time and true time ? 



CONVERSATION IX. 

ON THE MOON. 

Of the Moon's Motion ; Phases of the Moon ; Eclipses of 
the Moon ; Eclipses of Jupiter's Moons ; Of the Lati- 
tude and Longitude ; Of the Transits of the Inferiour 
Planets; Of the Tides. 

MRS. B. 

We shall to-day confine our attention to the moon, 
which offers many interesting phenomena. 

The moon revolves round the earth in the space of 
about twenty-nine days and a half, in an orbit nearly pa- 

512. In what tune does the moon revolve about the earth ? 



ON THE MOON. 125 

rallel to that of the earth, and accompanies us in our revo- 
lution round the sun. 

Emily, Her motion then must be rather of a compli- 
cated nature ; for as the earth is not stationary, but ad- 
vances in her orbit whilst the moon goes round her, the 
moon must proceed in a sort of progressive circle. 

Mrs. B. That is true ; and there are also other cir- 
cumstances which interfere with the simplicity and regu- 
larity of the moon's motion, but which are too intricate 
for you to understand at present. 

The moon always presents the same face to us, by 
which it is evident that she turns but once upon her axis, 
whilst she performs a revolution round the earth ; so that 
the inhabitants of the moon have but one day and one 
night in the course of a lunar month. 

Caroline. We afford them however the advantage of a 
magnificent m^on to enlighten their long nights. 

Mrs, B. That advantage is but partial ; for since we 
always see the same hemisphere of the moon, the inhabi- 
tants of that hemisphere alone can perceive us. 

Caroline. One half of the moon then enjoys our light 
every night, while the other half has constantly nights of 
darkness. If there are any astronomers in those regions, 
they would doubtless be tempted to visit t^e other hemi- 
sphere, in order to behold so grand a luminary as we 
must appear to them. But, pray, do they see the earth 
under all the changes which the moon exhibits to us ? 

Mrs. B. Exactly so. These changes are called the 
phases of the moon, and require some explanation. In 
figure 2, plate XI. let us say that S represents the sun, 
E the earth, and A B C D the moon in different parts of 
I her orbit. When the moon is at A, her dark side being 
, turned towards the earth, we shall not see her as at a ; but 
her disappearance is of very short duration, and as she ad- 
vances in her orbit we perceive her under the form of a 
new moon ; when she has gone through one-eighth of her 
i orbit at B, one quarter of her enlightened hemisphere 

1513. In what time does the moon turn on its axis ? 514. 
How is it known how long it takes the moon to revolve on its axis ? 

i 515. What is the length of the days and nights at the moon ? 

i 516. Does the earth exhibit the same changes to the moon, 

| that the moon exhibits to the earth? 517. What are the 

, changes of the moon called ? 18. How would you explain 

these changes bv the figure ? 

ii * 



126 ON THE MOON. 

will be turned towards the earth, and she will then appea 
horned as at b ; when she has performed one quarter of 
her orbit, she shows us one half of her enlightened side as 
at c; at d she is said to be gibbous, and at e the whole 
of the enlightened side appears to us, and the moon is at 
full. As she proceeds in her orbit she becomes again 
gibbous, and her enlightened hemisphere turns gradually 
away from us until she completes her orbit and disap- 
pears, and then again resumes her form of a new moon. 

When the moon is at full, or a new moon, she is said 
to be in conjunction with the sun, as they are then both 
in the same direction with regard to the earth ; when at 
her quarters she is said to be in opposition to the sun. 

Emily. Are not the eclipses produced by the moon 
passing between the sun and the earth ? 

Mrs. B. Yes ; when the moon passes between the 
sun and the earth, she intercepts his rays, or in other 
words, casts a shadow on the earth, then the sun is eclip- 
sed, and the day-light gives place to darkness, while the 
moon's shadow is passing over us. 

When, on the contrary, the earth is between the sun 
and the moon, it is we who intercept the sun's rays, and 
cast a shadow on the moon ; the moon is then darkened, 
she disappears from our view, and is eclipsed. 

Emily. But as the moon goes round the earth every 
month she must be once during that time between the 
earth and the sun, and the earth must likewise be once 
between the sun and the moon, and yet we have not a 
solar and a lunar eclipse every month. 

Mrs. B. The orbits of the earth and moon are not 
exactly parallel, but cross or intersect each other ; and 
the moon generally passes either above or below the 
earth when she is in conjunction with the sun, and does 
not therefore intercept the sun's rays, and produce an 
eclipse ; for this can take place only when the earth and 
moon are in conjunction in that part of their orbits which 



519. When is the moon said to be gibbous, and when horned ? 

520. When is the moon said to be in conjunction with the 

sun ? 521. When is the moon said to be in opposition to the 

sun ? 522. What causes an eclipse of the sun ? 523. How 

is an eclipse of the moon caused ? 524. As the moon passes 

between the sun and the earth, and as the earth passes between the 
sun and the moon, once every month, why do we not have a lu« 
nar and solar eclipse every month ? 



ON THE MOON. 



127 



cross each other, (called the nodes of their orbits,) because 
it is then only, that they are both in a right line with the 



sun 



II. -i 1 T 

Emily. And a partial eclipse takes place, 1 suppose, 
when the moon, in passing by the earth, is not sufficiently 
above or below the earth's shadow entirely to escape it 1 

Mrs. B. Yes, one edge of her disk then dips into the 
shadow, and is eclipsed ; but as the earth is larger than 
the moon, when the eclipse happens precisely at the 
nodes, they are not only total, but last for some length ot 

time. , _ _ _ 

When the sun is eclipsed, the total darkness is con- 
fined to one particular part of the earth, evidently show- 
ing that the moon is smaller than the earth, since she can- 
not entirely screen it from the sun. In fig. 1. plate Xll. 
you will find a solar eclipse described ; S is the sun, M 
the moon, and E the earth ; and the moon's shadow, you 
see, is not large enough to cover the earth. The lunar 
eclipses on the contrary are visible from every part of the 
earth, where the moon is above the horizon ; and we dis- 
cover by the length of time which the moon is in passing 
through the earth's shadow, that it would be sufficient to 
eclipse her totally, were she 47 times her actual size ; 
it follows, therefore, that the earth is 47 times the size oi 

the moon. m . , P , 

In fig. 2, S represents the sun, which pours forth rays 
of light in straight lines in every direction. E is the 
earth, and M the moon. Now a ray of light coming from 
one extremity of the sun's disk in the direction A B, will 
meet another coming from the opposite extremity in the 
direction C B ; the shadow of the earth cannot therefore 
extend beyond B ; as the sun is larger than the earth, the 
shadow of the latter is conical, or the figure of a sugar 
loaf; it gradually diminishes, and is much smaller than 
the earth where the moon passes through it, and yet we 

525. When does a partial eclipse take place ?- — 526. What 
is the consequence when an eclipse happens precisely at the nodes . 

527 When the sun is eclipsed does the total darkness extend 

to the whole hemisphere ? 528. What is shown from the dark- 
ness being confined to a particular spot of the earth ?— M#. 

Bv which figure is a solar eclipse illustrated ?- 630. How can 

the comparative size of the earth and moon be determined by a 

lunar eclipse ? 531. How much larger is the earth than the 

moon thus found to be ? 532. How does figure 2, plate Xll. 

illustrate this subject > 



28 ON THE MOON. 

find the moon to be not only totally eclipsed, but some 
length of time in darkness, and hence we are enabled to 
ascertain its real dimensions. 

Emily. When the moon eclipses the sun to us, we 
must be eclipsed to the moon ? 

Mrs. B. Certainly ; for if the moon intercept the 
sun's rays, and cast a shadow on us, we must necessarily 
disappear to the moon, but only partially, as in fig. 1. 

Caroline. There must be a great number of eclipses 
in the distant planets, which have so many moons. 

Mrs. B. Yes, few days pass without an eclipse taking 
place ; for among the number of satellites, one or other 
of them are continually passing either between their pla- 
net and the sun, or between the planet and each other. 
Astronomers are so well acquainted with the motion of 
the planets and their satellites, that they have calculated 
not only the eclipses of our moon, but those of Jupiter, 
with such perfect accuracy, that it has afforded a means 
of ascertaining the longitude. 

Caroline. But is it not very easy to find both the lati- 
tude and longitude of any place by a map or globe 1 

Mrs. B. If you know where you are situated, there 
is no difficulty in ascertaining the latitude or longitude of 
the place by referring to a map ; but supposing that you 
had been a length of time at sea, interrupted in your 
course by storms, a map would afford you very little as- 
sistance in discovering where you were. 

Caroline. Under such circumstances, I confess I 
should be equally at a loss to discover either latitude or 
longitude. 

Mrs. B. The latitude may be easily found by taking 
the altitude of the pole ; that is to say, the number of 
degrees that it is elevated above the horizon, for the pole 
appears more elevated as we approach it, and less as 
we recede from it. 

Caroline. But unless you can see the pole, how can 
you take its altitude ? 

Mrs. B. The north pole points constantly towards 
one particular part of the heavens in which a star is situ- 
ated, called the Polar Star :this star is visible on clear 

533. When is the earth eclipsed to the moon ? 534. Which 

figure illustrates the manner in which the earth is eclipsed to the 

moon ? 535. Are not the eclipses of the distant planets, which 

have so many moons, frequent ? 536. What benefit do we de 

rive from these eclipses ? 



ON THE MOON. 129 

nights from every part of the northern hemisphere ; the 
altitude of the polar star is therefore the same number 
of degrees as that of the pole : the latitude may also be 
determined by observations made on the sun or any of 
the fixed stars ; the situation therefore of a vessel at sea, 
with regard to north and south, is easily ascertained. 
The difficulty is respecting east and west, that is to say, 
its longitude. As we have no eastern poles from which 
we can reckon our distance, some particular spot must 
be fixed upon for that purpose. The English reckon 
from the meridian of Greenwich, where the royal obser- 
vatory is situated ; in French maps you will find that the 
longitude is reckoned from Paris. 

The rotation of the earth on its axis in 24 hours from 
west to east occasions, you know, an apparent motion of 
the sun and stars in the contrary direction, and the sun 
appears to go round the earth in the space of 24 hours, 
passing over fifteen degrees or a twenty-fourth part of 
the earth's circumference every hour ; therefore, when 
it is twelve o'clock in London, it is one o'clock in any 
place situated fifteen degrees to the east of London, as 
the sun must have passed the meridian of that place an 
hour before he reaches that of London. For the same 
reason it is eleven o'clock to any place situated fifteen 
degrees to tne west of London, as the sun will not come 
to that meridian till an hour later. 

If then the captain of a vessel at sea could know pre- 
cisely what was the hour at London, he could, by looking 
at his watch, and comparing it with the hour of the spot 
in which he was, ascertain the longitude. 

Emily. But if he had not altered his watch, since he 
sailed from London, it would indicate the hour it was 
then in London. 

Mrs. B. True ; but in order to know the hour of the 
day of the spot in which he is, the captain of a vessel re- 
gulates his watch by the sun when it reaches the meridian. 



537. How can the latitude of a place be determined ? 538. 

Why is it more difficult to determine, by observation, the longitude 
than the latitude of a place ? 539. From what place do the En- 
glish reckon longitude ? 540. What does the rotation of the 

earth upon its axis in 24 hours from west to east occasion? 

541. Over how many degrees does the sun thus appear to move 
every hour > 



J 30 ON THE MOON, 

Emily. Then if he had two watches, he might keep 
one regulated daily, and leave the other unaltered ; the 
former would indicate the hour of the place in which he 
was situated, and the latter the hour of London ; and by 
comparing them together, he would be able to calculate 
his longitude. 

Mrs. B. You have discovered, Emily, a mode of find- 
ing the longitude, which I have the pleasure to tell you, 
is universally adopted : watches of a superiour construc- 
tion, called chronometers, or time-keepers, are used for 
this purpose ; but the best watches are liable to imperfec- 
tions, and should the time-keeper go too fast, or too slow, 
there would be no means of ascertaining the errour ; im- 
plicit reliance cannot consequently be placed upon them. 
Recourse is therefore had to the eclipses of Jupiter's 
satellites. A table is made of the precise time at which 
the several moons are eclipsed to a spectator at London ; 
when they appear eclipsed to a spectator in any other 
spot, he may, by consulting the table, know what is the 
hour at London ; for the eclipse is visible at the same 
moment from whatever place on the earth it is seen. 
He has then only to look at the watch which points out 
the hour of the place in which he is, and by observing the 
difference of time there, and at London, he may immedi- 
ately determine his longitude. 

Let us suppose, that a certain moon of Jupiter is al- 
ways eclipsed at six o'clock in the evening ; and that a 
man at sea consults his watch, and finds that it is ten 
o'clock, at night, where he is situated, at the moment the 
eclipse takes place ; what will be his longitude 1 

Emily. That is four hours later than in London : four 
times fifteen degrees makes 60 ; he would, therefore, be 
sixty degrees east of London, for the sun must have pass- 
ed his meridian before it reaches that of London. 

Mrs. B. For this reason the hour is always later than 
in London, when the place is east longitude, and earlier 
when it is west longitude. Thus the longitude can be 

542. How can longitude be determined at sea by the use of two 

watches ? 543. What is the difficulty in depending at sea on 

this mode of finding the longitude ? 544. How can longitude 

be determined from the eclipses of Jupiter's satellites ? 545. 

What case is supposed to illustrate this method of findirig the 

longitude of a place ? 54C How is east known from west 

longitude when thus found ? 



ON THE MOON. 131 

ascertained whenever the eclipses of Jupiter's moons are 
visible. 

But it is not only the secondary planets which produce 
eclipses, for the primary planets near the sun eclipse him 
to those at a greater distance when they come in conjunc- 
tion in the nodes of their orbits ; but as the primary 
planets are much longer in performing their course round 
the sun, than the satellites in going round their primary 
planets, these eclipses very seldom occur. Mercury and 
Venus have however passed in a right line between us 
and the sun, but being at so great a distance from us, 
their shadows did not extend so far as the earth ; no 
darkness was therefore produced on any part of our globe ; 
but the planet appeared like a small black spot, passing 
across the sun's disk ; this is called a transit of the planet. 
It was by the last transit of Venus, that astronomers 
were enabled to calculate with some degree of accuracy 
! the distance of the earth from the sun, and the dimensions 
of the latter. 

, Emily. I have heard that the tides are effected by the 
moon, but I cannot conceive what influence it can have 
on them. 

Mrs. B. They are produced by the moon's attraction 
which draws up the waters in a protuberance. 

Caroline. Does attraction act on water more power- 
fully than on land 1 I should have thought it would have 
been just the contrary, for land is certainly a more dense 
body than water 1 

Mrs. B. Tides do not arise from water being more 
strongly attracted than land, for this certainly is not the 
; case ; but the cohesion of fluids being much less than 
- that of solid bodies, they more easily yield to the power 
of gravity, in consequence of which the waters immedi- 
ately below the moon are drawn up by it in a protube- 
j ranee, producing a full tide, or what is commonly called 
; high water, at the spot where it happens. So far the 
I theory of the tides is not difficult to be understood. 



547. Why do the distant primary planets eclipse the sun less 

frequently than do their satellites ? 548. What is meant by 

the transit of a planet ? 549. What use was made by astrono- 
mers of the transit of Venus ? 550. What occasions the tides I 

551. How can the tides be occasioned by the attraction of 

the moon . unless water is acted on more powerfully by gravita- 
tion than the land p 



132 ON THE MOON. 

Caroline, On the contrary, nothing can be more sim- 
ple ; the waters, in order to rise up under the moon, must 
draw the waters from the opposite side of the globe, and 
occasion ebb-tide, or low water in those parts. 

Mrs. B. You draw your conclusion rather too hastily, 
my dear; for according to your theory, we should have 
full tide only once in twenty-four hours, that is, every 
time that we were below the moon, while we find that we 
have two tides in the course of twenty-four hours, and 
that it is high water with us and with our antipodes at 
the same time. 

Caroline. Yet it must be impossible for the moon to 
attract the sea in opposite parts of the globe, and in oppo- 
site directions at the same time. 

Mrs. B. This opposite tide is rather more difficult to 
explain, than that which is drawn up beneath the moon ' f 
with a little attention, however, I hope I shall be able to 
make you understand it. 

You recollect that the earth and moon are mutually at- 
tracted towards a point, their common centre of gravity 
and of motion ; can you tell me what it is that prevents 
their meeting and uniting at this point 1 

Emily. Their projectile force, which gives them a 
tendency to fly from this centre. 

Mrs. B. And is hence called their centrifugal forcer 
Now we know that the centrifugal force increases in pro- 
portion to the distance from the centre of motion. 

Caroline. Yes, I recollect your explaining that to us, 
and illustrating it by the motion of the flyers of a wind- 
mill, and the spinning of a top. 

Emily. And it was but the other day you showed us 
that bodies weighed less at the equator, than in the polar re- 
gions, in consequence of the increased centrifugal force 
in the equatorial parts. 

Mrs. B. Very well. The power of attraction, on 
the contrary, increases as the distance from the centre of 
gravity diminishes ; when, therefore, the two centres of 
gravity and of motion are in the same spot, as is the case 
with regard to the moon and the earth, the centrifugal 

552. How often do we have a high tide ? 553. What pre- 
vents the earth and moon from being drawn together in their com- 
mon centre of gravity ? 554. In what proportion does the 

centrifugal force increase ? 555. In what ratio does the pow- 
er of attraction increase ? 



ON THE MOON. 133 

power and those of attraction, will be in inverse propor- 
tion to each other ; that is to say, where the one is strong- 
est, the other will be weakest. 

Emily- Those parts of the ocean, then, which are 
most strongly attracted, will have least centrifugal force ; 
and those parts which are least attracted, will have the 
greatest centrifugal force. 

Mrs, B. In order to render the question more simple, 
let us suppose the earth to be every where covered by the 
ocean, as represented in (fig. 3, pi. XII.) M is the moon, 
ABCD the earth, and X the common centre of gravity 
and of motion of these two planets. Now the waters on 
the surface of the earth, about A, being more strongly 
attracted than any other part, will be elevated ; the at- 
traction of the moon at B and C being less, and at D least 
of all. But the centrifugal force at D, will be greatest, 
and the waters there, will in consequence have the great- 
est tendency to recede from the moon ; the waters at B 
and C will have less tendency to recede, and at A least 
of all. The waters, therefore, at D, will recede furthest, 
at the same time that they are least attracted, and in con- 
sequence will be elevated in a protuberance similar to that 
at A. 

Emily, The tide A, then, is produced by the moon's 
attraction, and increased by the feebleness of the centri- 
fugal power in those parts ; and the tide D is produced 
by the centrifugal force, and increased by the feebleness 
of the moon's attraction in those parts.* 



* The opinion of Mrs. Marcet concerning the tide on that part of 
the earth furthest from the mcon is not universally, and it is be- 
• lieved, not generally adopted by writers on this subject. The 
theory may be an ingenious one ; but, it seems more probable, 
that the centrifugal motion of the earth is only an auxiliary and 
not a principal cause of this tide ; and that its principal cause is 
the moon's attraction. For if the globe were one solid mass of 
matter, every part of it would be drawn alike towards the moon \ 



556. How would you explain the production of the tides by 
Figure 3, plate XII ? 557. Is the opinion of Mrs. Bryan con- 
cerning the tides, universally adopted f 558. What is thought 

a more probable cause ofthetide upon the part of the earth furthest 
from the moon than the centrifugal motion of the earth f 
12 



181 ON THE MOON. 

Caroline. And when it is high water at A and D, it is 
low water at B and C : now I think I comprehend the 
nature of the tides again, though I confess it is not quite 
so easy as I at first thought. 

But, Mrs. B., why does not the sun produce tides as 
well as the moon ; for its attraction is greater than that of 
the moon 1 

Mrs. B. It would be, at an equal distance, but our 
vicinity to the moon makes her influence more powerful. 
The sun has, however, a considerable effect on the tides, 
and increases or diminishes them as it acts in conjunction 
with, or in opposition to the moon. 

Emily. I do not quite understand that. 

Mrs. B. The moon is a month in going round the 
earth ; twice during that time, therefore, at full and at 
change, she is in the same direction as the sun, both then 
act in conjunction on the earth, and produce very great 
tides, called spring tides, as described in fig. 4. at A and 
B ; but when the moon is at the intermediate parts of her 
orbit, the sun, instead of affording assistance, weakens 
her power by acting in opposition to it ; and smaller tides 
are produced, called neap tides, as represented in fig. $•* 



DUt as there is not a sufficient degree of cohesive attraction in the 
watery parts of it to preserve perfectly its form, the waters upon 
that part of it nearest the moon are drawn away from the land, 
while the land, which is supposed to constitute the central regions 
of the globe, is drawn away from the waters upon that part of it 
most distant from the moon. 



* Although the spring and neap tides are produced by the con- 
junction and opposition of the sun and moon, yet their effects are 
not immediate ; the highest tides happen not on the days of the 
full and change, neither do the lowest tides happen on the days 



558. Hoio could you account for this tide, if produced hy the 

moons attraction ? 559. As the sun is larger than the moon, 

why does not the sun produce the chief influence in the production 

of the tides? 560. But does the sun exercise no influence in 

the production of the tides ? 561. When does it increase, and 

when diminish the tides? 562. What is meant by the sun 

and moon acting in conjunction on the tides ? 563. What are 

the spring tides ? 564. What are the tides called when the 

sun and moon are in opposition ? 565. How would you explain 

the spring and neap tides by the Figures '- 



ON THE MOON. 135 

Emily. 1 have often observed the difference of these 
tides when I have been at the sea side. 

But since attraction is mutual between the moon and 
the earth, we must produce tides in the moon ; and these 
must be more considerable in proportion as our planet is 
larger. And yet the moon does not appear of an oval 
form. 

Mrs. B. You must recollect, that in order to render 
the explanation of the tides clearer, we suppose the whole 
surface of the earth to be covered with the ocean ; but 
that is not really the case, either with the earth or the 
moon, and the land which intersects the water destroys 
the regularity of the effect. 

Caroline. True ; we may, however, be certain, that 
whenever it is high water the moon is immediately over 
our heads. 

Mrs. B. Not so either ; for as a similar effect is pro- 
duced on that part of the globe immediately beneath the 
moon, and on that part most distant from it, it cannot be 
over the heads of the inhabitants of both those situations 
at the same time. Besides, as the orbit of the moon is 
very nearly parallel to that of the earth, she is never ver- 
tical but to the inhabitants of the torrid zone ; in that 
climate, therefore, the tides are greatest, and they dimi- 
nish as you recede from it and approach the poles. 

Caroline. In the torrid zone, then, I hope you will 
grant that the moon is immediately over, or opposite the 
spots where it is high water ? 

Mrs. B. I cannot even admit that ; for the ocean na- 
turally partaking of the earth's motion, in its rotation from 
west to east, the moon, in forming a tide, has to contend 



of quadratures. But on account of the continuation of motion, it is, 
some time after, the exercise of the sun and moon's attraction, in 
the manner supposed, that the effect of their forces is most to be 
seen. So that the greatest spring tides commonly happen three 
days after the new and full moons ; and the least neap tides three 
days after the first and third quarters. 



566. How much after the conjunction and opposition of the sun 

and moon do the spring and neap tides take place t 567- In 

what parts of the earth are the tides highest ? 568. Why are 

they highest in the equatorial regions ; 



136 ON THE MOON. 

against the eastern motion of the waves. All matter, you 
know, by its inertia, makes some resistance to a change 
of state ; the waters, therefore, do not readily yield to the 
attraction of the moon, and the effect of her influence is 
not complete till three hours after she has passed the me- 
ridian, where it is full tide. 

Emily, Pray what is the reason that the tide is three 
quarters of an hour later every day ? 

Mrs. B. Because it is twenty-four hours and three- 
quarters before the same meridian on our globe returns 
beneath the moon. The earth revolves on its axis in 
about twenty-four hours ; if the moon were stationary, 
therefore, the same part of our globe would, every twen- 
ty-four hours, return beneath the moon ; but as during 
our daily revolution the moon advances in her orbit, the 
earth must make more than a complete rotation in order to 
bring the same meridian opposite the moon : we are three 
quarters of an hour in overtaking her. The tides, there- 
fore, are retarded for the same reason that the moon rises 
later by three quarters of an hour every day.* 

We have now, I think, concluded the observations 1 
had to' make to you on the subject of astronomy ; at our 
next interview, I shall attempt to explain to you the ele- 
ments of bydrostaticks. 

* There are no tides in lakes, because they are generally so 
small that when the moon is vertical she attracts every part alike ; 
and by rendering all the waters equally light, no part can be rais- 
ed higher than another. The Mediterranean and Baltick seas 
have very small elevations, because the inlets by which they com- 
municate with the ocean are so narrow, that they cannot in so 
short a time either receive or discharge enough, sensibly to raise 
or sink their surfaces ? 

5G9. Why is it not high water at a place, when the moon is di- 
rectly over the meridian of it ? 570. How long after the moon 

passes the meridian of a place before the effect of her influence 

becomes complete ? 571. Why are the tides three quarters of 

an hour later every day ? 572. Why are there no tides on the 

lakes ? 573. Why are the tides small in the Mediterranean 

and Baltick seas ? 



ON THE MECHANICAL PROPERTIES OF FLUIDS. 137 

CONVERSATION X. 

ON THE MECHANICAL PROPERTIES OF FLUIDS. 

Definition of a Fluid; Distinction between Fluids and 
Liquids ; Of Non-Elastick Fluids ; Scarcely suscepti- 
ble of Compression ; Of the Cohesion of Fluids ; Of 
their Gravitation ; Of their Equilibrium ; Of their 
Pressure; Of Specifick Gravity; Of the Specifick 
Gravity of Bodies heavier than Water ; Of those of 
the same Weight as Water ; Of those lighter than Wa- 
ter ; Of the Specifick Gravity of Fluids. 

MRS. B. 

We have hitherto confined our attention to the me- 
chanical properties of solid bodies, which have been illus- 
trated, and, I hope, thoroughly impressed upon your me- 
mory, by the conversations we have subsequently had on 
astronomy. It will now be necessary for me to give you 
some account of the mechanical properties of fluids — a 
science which is called hydrostaticks. A fluid is a sub- 
stance which yields to the slightest pressure. If you dip 
your hand into a basin of water, you are scarcely sensible 
of meeting with any resistance. 

Emily. The attraction of cohesion is, then, I suppose, 
less powerful in fluids than in solids 1 

Mrs. B. Yes ; fluids, generally speaking, are bodies 
of less density than solids. From the slight cohesion of 
the particles of fluids, and the facility with which they 
slide over each other, it is inferred, that they must be 
small, smooth, and globular ; smooth, because there ap- 
pears to be little or no friction among them ; and globu- 
lar, because touching each other but by a point would ac- 
count for the slightness of their cohesion.* 

* If the particles of fluids are round, there must be vacant spaces 
between them, in the same manner as there are vacuities between 
cannon balls that are piled together ; between these balls smaller 

574. What is the science called that treats of the mechanical 

properties of fluids ? 575. What is meant by a fluid ? 576. 

In which is the attraction of cohesion the most powerful, solids or 

fluids ? 577. What is inferred from the slight cohesion of the 

particles of fluids, and the facility with which they slide over each 
other ? 

12* 



138 ON TIIE MECHANICAL PROPERTIES OF FLUIDS. 

Caroline. Pray what is the distinction between a fluid 
and a liquid ? 

Mrs. B. Liquids comprehend only one class of fluids. 
There is another class distinguished by the name of elas- 
tick fluids, or gases, which comprehends the air of the 
atmosphere, and all the various kinds of air with which 
you will become acquainted when you study chemistry. 
Their mechanical properties we shall examine at our next 
meeting, and confine our attention this morning to those 
of liquids, or non-elastick fluids. 

Water and liquids in general, are scarcely susceptible 
of being compressed, or squeezed into a smaller space 
than that which they naturally occupy. This is supposed 
to be owing to the extreme minuteness of their particles, 
which, rather than submit to compression, force their 
way through the pores of the substance which confines 
them. This was shown by a celebrated experiment 
made at Florence many years ago. A hollow globe of 
gold was filled with water, and on its being submitted to 
great pressure, the water was seen to exude through the 
pores of the gold, which it covered with a fine dew. 
Fluids gravitate in a more perfect manner than solid 
bodies ; for the strong cohesive attraction of the particles 
of the latter in some measure counteracts the effects of 
gravity. In this table, for instance, the cohesion of the 
particles of wood enables four slender legs to support a 
considerable weight. Were the cohesion ..destroyed, or, 
in other words, the wood converted into a fluid, no sup- 
port could be afforded by the legs, for the particles no 



shot may be placed,and between these, other still smaller, or gravel, 
or sand, may be diffused. In a similar manner, a certain quantity 
of particles of sugar can be taken up in water without increasing 
its bulk, and when the water has dissolved the sugar, salt may be 
dissolved in it, and yet the bulk remain the same : and admitting 
that the particles of water are round, this is easily accounted for. 

578. What reason is given in the note for supposing that the 

particles of fluids are round? 579. What is the distinction 

between a liquid and a fluid ? 580. Are water and other liquids 

susceptible of compression ? 581. What is the reason for sup- 
posing they are not ? 582. What experiment has been made 

to show that liquids are not compressible ? 583. How do flu- 
ids gravitate compared with solids ? 584. What example is 

given to show that solids gravitate in a less perfect manner than 
liquids ? 



ON THE MECHANICAL PROPERTIES OF FLUIDS. 139 

longer cohering together, each would press separately 
and independently, and would be brought to a level with 
the surface of the earth. 

Emily. This want of cohesion is then the reason why 
fluids can never be formed into figures, or maintained in 
heaps ; for though it is true the wind raises water into 
waves, they are immediately afterwards destroyed by gra- 
vity, and water always finds its level. 

Mrs. B. Do you understand what is meant by the 
level, or equilibrium of fluids ? 

Emily. I believe I do, though I feel rather at a loss 
to explain it. Is not a fluid level when its surface is 
smooth and flat, as is the case with all fluids when in a 
state of rest. 

Mrs. B. Smooth, if you please, but not flat ; for the 
definition of the equilibrium of a fluid is, that every part 
of the surface is equally distant from the point to which 
gravity tends, that is to say, from the centre of the earth ; 
hence the surface of all fluids must be bulging, not flat, 
since they will partake of the spherical form of the globe. 
This is very evident in large bodies of water, such as the 
ocean, but the sphericity of small bodies of water is so 
trifling, that their surfaces appear flat. 

This level, or equilibrium of fluids is the natural re- 
sult of their particles gravitating independently of each 
other ; for when any particle of a fluid accidentally finds 
itself elevated above the rest, it is attracted down to the 
level of the surface of the fluid, and the readiness with 
which fluids yield to the slightest impression will enable 
the particle by its weight to penetrate the surface of the 
fluid and mix with it. 

Caroline. But I have seen a drop of oil float on the 
surface of water without mixing with it. 

Mrs. B. That is because oil is a lighter liquid than 
water. If you were to pour water over it, the oil would 
rise to the surface, being forced up by the superiour gravi- 
ty of the water. Here is an instrument called a water- 
level, (fig. 1, plate XIII.) which is constructed upon the 
principle of the equilibrium of fluids. It consists of a 

585. Why cannot liquids be moulded into figures like solids ? 
586. What is meant by the level or equilibrium of fluids ? 

587. Of what is the level or equilibrium of fluids the result ? 

588. Why will oil remain upon the top of water ? -589. How 

Is a water-level constructed ? 



140 ON THE MECHANICAL PROPERTIES OF FLUIDS. 

short tube, A B, closed at both ends, and containing a 
little water ; when the tube is not perfectly horizontal the 
water runs to the lower end, and it is by this means that 
the level of any situation to which we apply the instru- 
ment, is ascertained. 

Solid bodies you may, therefore, consider as gravitat- 
ing in masses, for the strong cohesion of their particles 
makes them weigh altogether, while every particle of a 
fluid may be considered as composing a separate mass, 
gravitating independently of each other. Hence the re- 
sistance of a fluid is considerably less than that of a solid 
body ; for the resistance of the particles acting separate- 
ly, they are more easily overcome. 

Emily. A body of water, in falling, does certainly less 
injury than a solid body of the same weight. 

Mrs. B. The particles of fluids acting thus indepen- 
dently, press against each other in every direction, not 
only downwards but upwards, and laterally or sideways ; 
and in consequence of this equality of pressure, every 
particle remains at rest in the fluid. If you agitate the 
fluid you disturb this equality of pressure, and the fluid 
will not rest till its equilibrum is restored. 

Caroline. The pressure downwards is very natural ; 
it is the effect of gravity, one particle weighing upon 
another presses on it ; but the pressure sideways, and 
particularly the pressure upwards, I cannot understand. 

Mrs. B. If there were no lateral pressure, water 
would not run out of an opening on the side of a vessel. 

If you fill a vessel with sand, it will not run out of such 
an opening, because there is scarcely any lateral pressure 
among its particles. 

Emily. When water runs out of the side of a vessel, 
is it not owing to the weight of the water above the 
opening ? 

Mrs. B. If the particles of fluids were arranged in 
regular columns thus, (fig. 2.) there would be no lateral 
pressure, for when one particle is perpendicularly above 
the other, it can only press it downwards ; but as it must 
continually happen, that a particle presses between two 
particles beneath, (fig. 3.) these last must suffer a lateral 
pressure. 

500. Why do solid bodies gravitate in masses ? 591. Why 

is the resistance of fluids less than that of solids ? 592. Why 

are fluids inclined to a state of rest or equilibrium ? 593. Why 

will liquids run out of an opening in the vessel containing them ? 



ON THE MECHANICAL PROPERTIES OF FLUIDS* 141 

Emily. The same as when a wedge is driven into 
a piece of wood, and separates the parts laterally. 

Mrs. B. Yes. The lateral pressure proceeds, there- 
fore, entirely from the pressure downwards, or the weight 
of the liquid above ; and consequently the lower the ori- 
fice is made in the vessel, the greater will be the velocity 
of the water rushing out of it. Here is a vessel of water 
(fig. 5.) with three stop cocks at different heights ; we 
shall open them, and you will see with what different de- 
grees of velocity the water issues from them. Do you un- 
derstand this, Caroline 7* 

Caroline. Oh yes. The water from the upper spout 
receiving but a slight pressure, on account of its vicinity 
to the surface, flows but gently ; the second cock having 
a greater weight above it, the water is forced out with 
greater velocity, whilst the lowest cock, being near the 
bottom of the vessel, receives the pressure of almost the 
whole body of water, and rushes out with the greatest 
impetuosity. 

Mrs. B. Very well ; and you must observe, that as 
the lateral pressure is entirely owing to the pressure down- 
wards, it is not effected by the horizontal dimensions of 
the vessel, which contains the water, but merely by its 
depth ; for as every particle acts independently of the 
rest, it is only the column of particles, immediately above 
the orifice, that can weigh upon and press out the water. 

Emily. The breadth and width of the vessel then can 
be of no consequence in this respect. The lateral pres- 
sure on one side, in a cubical vessel, is, I suppose, not so 
great as the pressure downwards. 



* An empty bottle being corked, and, by means of a weight, let 
down a certain depth into the sea, it will be broken, or the cork 
will be driven into it by the perpendicular pressure. But a bottle 
filled with water, or any other liquid, may be let down to any depth, 
without damage, because in this case the internal pressure is 
equal to the external ? 

594. ^ From what does the lateral pressure of liquids proceed ? 

595. How would you illustrate the lateral and downward 

pressure of fluids by the figures ? 596. What fact is mentioned 

in the note concerning the pressure of liquids ? 597. To what 

is the velocity of liquids, issuing from an orifice in the side of a 
vessel, proportional ? 



142 ON THE MECHANICAL PROPERTIESOF FLUIDS. 

Mrs* B. No, in a cubical vessel the pressure down- 
wards will be double the lateral pressure on one side ; fen 
every particle at the bottom of the vessel is pressed upon 
by a column of the whole depth of the fluid, whilst the 
lateral pressure diminishes from the bottom upwards to 
the surface, where the particles have no pressure. 

Caroline, And from whence proceeds the pressure of 
fluids upwards ? that seems to me the most unaccounta- 
ble, as it is in direct opposition to gravity. 

Mrs. B. And yet it is a consequence of their pres- 
sure downwards. When, for example, you pour water into 
a tea-pot, the water rises in the spout to a level with the 
water in the pot. The particles of water at the bottom 
of the pot are pressed upon by the particles above them ; 
to this pressure they will yield, if there is any mode of 
making way for the superiour particles, and as they can- 
not descend, they will change their direction and rise in 
the spout. 

Suppose the tea-pot to be filled with columns of parti- 
cles of water similar to that described in fig. 4. the par- 
ticle 1 at the bottom will be pressed laterally by the par 
tide 2, and by this pressure be forced into the spout 
where, meeting with the particle 3, it presses it upwards, 
and this pressure will be continued, from 3 to 4, from 4 
to 5, and so on till the water in the spout has risen to a 
level with that in the pot. 

Emily. If it were not for this pressure upwards, forc- 
ing the water to rise in the spout, the equilibrium of the 
fluid would be destroyed. 

Caroline. True ; but then a tea-pot is wide and large, 
and the weight of so great a body of water as the pot will 
contain, may easily force up and support so small a quan- 
tity as will fill the spout. But would the same effect be 
produced if the spout and the pot were of equal dimen- 
sions 1 

Mrs. B. Undoubtedly it would. You may even re- 
verse the experiment by pouring water into the spout, and 
you will find that water will rise in the pot to a level 
with that in the spout ; for the pressure of the small 

598. How does the pressure downwards, in a cubical vessel, 

compare with the lateral pressure ? 599. Whence proceeds 

the pressure of liquids upwards ? 600. How would you illus- 
trate, from the figure, the upward pressure of liquids occasioned 

by the downward pressure ? 601 . What will be the effect, in 

relation to this subject, if water is poured into the spout ? 



ON THE MECHANICAL PROPERTIES OF FLUIDS. 143 

quantity of water in the spout will force up and support 
the larger quantity in the pot. In the pressure upwards, 
as well as that laterally, you see that the force of pressure 
depends entirely on the height, and is quite independent 
of the horizontal dimensions of the fluid. 

As a tea-pot is not transparent, let us try the experi- 
ment by filling this large glass goblet by means of this nar 
row tube. (fig. 6.) 

Caroline. Look, Emily, as Mrs. B. fills it, how the 
i water rises in the goblet, to maintain an equilibrium with 
\ that in the tube. 

Now, Mrs. B., will you let me fill the tube by pouring 
I water into the goblet. 

Mrs. B. That is impossible. However, you may try 
f the experiment, and I doubt not but that you will be able 

to account for its failure. 
bj Caroline. It is very singular, that if so small a co- 
• lumn of water as is contained in the tube can force up and 
1 support the whole contents of the goblet ; that the weight 
; of all the water in the goblet should not be able to force 
( up the small quantity required to fill the tube : — oh, I see 
i now the reason, the water in the goblet cannot force that 
! in the tube above its level ; and as the end of the tube is 
J considerably higher than the goblet, it can never be filled 
by pouring water into the goblet. 

Mrs. B. And if you continue to pour water into the 
goblet when it is full, the water will run over instead of 
i rising above the level in the tube. 

I shall now explain to you the meaning of the specifick 
i gravity of bodies. 

Caroline. What ! is there another species of gravity 
ii with which we are not yet acquainted ? 
( Mrs. B. t No ; the specifick gravity of a body, means 
I simply its weight compared with that of another body of 
the same size. When we say, that substances, such as 
lead and stones-are heavy, and that others, such as paper 
! and feathers, are light, we speak comparatively ; that is 
I to say, that the first are heavy, and the latter light, in 
1 comparison with the generality of substances in nature, 
| Would you call wood and chalk light or heavy bodies 1 

602. What is the object of figure 6, plate XIII. ? 603. What 

I is meant by the specifick gravity of bodies ? 604. When we 

say that such bodies as lead and stones are heavy, and that such 
as paper and feathers are light, how do we speak ? 



144 ON THE MECHANICAL PROPERTIES OF FLUIDS, 

Caroline. Some kinds of wood are heavy, ceitainly, 
as oak and mahogany ; others are light, as deal and box. 

Emily. I think I should call wood in general a heavy 
body, for deal and box are light only in comparison to 
wood of a heavier description. I am at a loss to deter- 
mine whether chalk should be ranked as a heavy or a 
light body ; I should be inclined to say the former, if it 
were not that it is lighter than most other minerals. I 
perceive that we have but vague notions of light and heavy. 
I wish there was some standard of comparison, to which 
we could refer the weight of all other bodies. 

Mrs. B. The necessity of such a standard has been 
so much felt, that a body has been fixed upon for this 
purpose. What substance do you think would be best 
calculated to answer this end 1 

Caroline. It must be one generally known and easily 
obtained, lead or iron for instance. 

Mrs. B. All the metals expand by heat, and condense 
by cold. A piece of lead, let us say a cubick inch for in- 
stance, would have less specifick gravity in summer than 
in winter ; for it would be more dense in the latter season. 

Caroline. But, Mrs. B., if you compare the weight of 
equal quantities of different bodies, they will all be alike. 
You know the old saying that a pound of feathers is as 
heavy as a pound of lead. 

Mrs. B. When therefore w r e compare the weight of 
different kinds of bodies, it would be absurd to take quan- 
tities of equal weight, we must take quantities of equal 
bulk ; pints or quarts, not ounces or pounds. 

Caroline. Very true ; I perplexed myself by thinking 
that quantity referred to weight, rather than to measure. 
It is true, it would be as absurd to compare bodies of the 
same size in order to ascertain which was largest, as to 
compare bodies of the same weight in order to discover 
which was heaviest. 

Mrs. B. In estimating the specifick gravity of bodies, 
therefore, we must compare equal bulks, and we shall 
find that their specifick gravity will be proportional to their 

605. Why would not metals, as lead, or iron, answer for the 

standard to determine the specifick gravities of bodies ? 606. 

What body has been adopted as a standard of reference ? 



ON THE MECHANICAL PROPERTIES OF FLUIDS. 145 

weights. The body which has been adopted as a stand- 
ard of reference is distilled water.* 

Emily. I am surprised that a fluid should have been 
chosen for this purpose, as it must necessarily be contain- 
ed in some vessel, and the weight of the vessel will re- 
quire to be deducted. 

Mrs. B. In order to learn the specifick gravity of a 
solid body, it is not necessary to put a certain measure 
of it in one scale, and an equal measure of water into 
the other scale : but simply to weigh the body under trial 
in water. If you weigh a piece of gold in a glass of water, 
will not the gold displace just as much water, as is equal 
to its own bulk ] 

Caroline. Certainly, where one body is, another can- 
not be at the same time ; so that a sufficient quantity of 
water must be removed, in order to make way for the 
gold. 

Mrs. B. Yes, a cubick inch of water to make room 
for a cubick inch of gold ; remember that the bulk alone 
is to be considered, the weight has nothing to do with the 
quantity of water displaced, for an inch of gold does not 



* The method of ascertaining the specifick gravities of bodies was 
discovered accidentally by Archimedes. He had been employed 
by the king of Syracuse to investigate the metals of a golden crown 
which he suspected had been adulterated by the workmen. The 
philosopher laboured at the problem in vain, till going one day into 
the bath, he perceived that the water rose in the bath in proportion 
to the bulk of his body ; he instantly perceived that any other sub- 
stance of equal size would have raised the water just as much, 
though one of equal weight and less bulk could not have produced 
the same effect. He then got two masses, one of gold and one of 
silver, each equal in weight to the crown, and having filled a ves^ 
sel very accurately with water, he first plunged the silver mass into 
^ it, and observed the quantity of water that flowed over ; he then 
did the same with the gold, and found that a less quantity had pass- 
ed over than before. Hence he inferred tnat, though of equal 
weight, the bulk of the silver was greater than that of the gold, and 
that the quantity of water displaced was, in each experiment, equal 
to the bulk of the metal. He next made trial with the crown, and 
found it displaced more water than the gold, and less than the sil- 
ver, which led him to conclude, that it was neither pure gold nor 
pure silver. 



607. Who discovered the method of ascertaining the specifick 
gravities of bodies f 608. What led him to make the discovery ? 

13 



146 ON THE MECHANICAL PROPERTIES OF FLUIDS. 

occupy more space, and therefore will not displace more 
water than an inch of ivory, or any other substance that 
will sink in water. 

Well, you will perhaps be surprised to hear that the 
gold will weigh less in water, than it did out of it. 

Emily. And for what reason ? 

Mrs. B. On account of the upward pressure of the 
particles of water, which in some measure supports the 
gold, and by so doing diminishes its weight. If the body 
immersed in water was of the same weight as that fluid, 
it would be wholly supported by it, just as the water which 
it displaces was supported previous to its making way for 
the solid body.. If the body is heavier than the water, it 
cannot be wholly supported by it ; but the water will offer 
some resistance to its descent. 

Caroline. And the resistance which water offers to the 
descent of heavy bodies immersed in it, (since it proceeds 
from the upward pressure of the particles of the fluid,) 
must, in all cases, I suppose, be the same. 

Mrs. B. Yes ; the resistance of the fluid is propor- 
tioned to the bulk, and not to the weight of the body im- 
mersed in it ; all bodies of the same size, therefore, lose 
the same quantity of their weight in water. Can you form 
any idea what this loss will be ? 

Emily. I should think it would be equal to the weight 
of the water displaced ; for, since that portion of the wa- 
ter was supported before the immersion of the solid body, 
an equal weight of the solid body will be supported. 

Mrs. B. You are perfectly right : a body weighed in 
water loses just as much of its weight, as is equal to that 
of the water it displaces : so that if you were to put the 
water displaced into the scale to which the body is sus- 
pended, it would restore the balance. 

You must observe, that when you weigh a body in 
water, in order to ascertain its specifick gravity, you must 
not sink the basin of the balance in the water ; but either 
suspend the body to a hook at the bottom of the basin, 
or else take off the basin, and suspend it to the arm of 
the balance, (fig. 7.) Now suppose that a cubick inch 

009. Why does a body weigh less in the water than out of it ? 

-010. To what is the resistance of water to a body immersed 

in it proportioned ? 611. How much does a body weighed in 

the water lose of its weight ? (>12. Which figure shows the 

manner of weighing a body in water ? 



ON THE MECHANICAL PROPERTIES OF FLUIDS. 147 

of gold weighed 19 ounces out of water" and lost one 
ounce of its weight by being weighed in water, what would 
be its specifick gravity 1 

Caroline. The cubick inch of water it displaced must t 
weigh that one ounce ; and as a cubick inch of gold 
weighs 19 ounces, gold is 19 times as heavy as water. 

Emily. I recollect having seen a table of the com- 
parative weights of bodies, in which gold appeared to me 
to be estimated at 19 thousand times the weight of water. 

Mrs. B. You misunderstood the meaning of the table. 
In the estimation you allude to, the weight of water was 
reckoned at 1000. You must observe, that the weight 
of a substance, when not compared to that of any other, 
is perfectly arbitrary ; and when water is adopted as a 
standard, we may denominate its weight by any number 
we please ; but then the weight of all bodies tried by this 
standard must be signified by proportional numbers. 

Caroline. We may call the weight of water for exam- 
ple, one, and then that of gold would be nineteen ; or if 
we choose to call the weight of water 1000, that of gold 
would be 19,000. In short, the specifick gravity means 
how much more a body weighs than an equal bulk of 
water. 

Mrs. B. It is rather the weight of a body compared 
with that of water ; for the specifick gravity of many 
substances is less than that of water.* 



* 


Specifick 


Gravities of Various Bodies. 




Fine gold 




19,640 


Mahogany 


1,063 


Lead - 




11,325 


Milk 


1,034 


Fine Silver 




11,091 


Rain water 


1,000 


Copper 




9,000 


Oil - - 


,920 


Iron 




7,645 


Ice 


,908 


Marble 




2,705 


Brandy - 


,920 


Glass - 




3,000 


Living men 


,891 


Chalk - 




1,793 


Cork 


,240 


Coal 


- 


1,250 


Common air - 


,112 



Experiments have been made to ascertain the specifick gravity of 
living men, in order to know what weight of cork fastened to a per- 
son in the water will keep him from sinking, on the supposition 
that most men were specifically heavier than river water ; but, con- 
trary to expectation, it was found from trials made upon ten diffe- 

613. What is the specifick gravity of living men $ 



148 ON THE MECHANICAL PROPERTIES OF FLUIDS. 

Caroline. Then you cannot ascertain the specific!* 
gravity of such substances in the same manner as that of 
gold ; for a body that is lighter than water will float on its 
surface without displacing any water. 

Mrs, B. If a body were absolutely light, it is true 
that it would not displace a drop of water ; but the bodies 
we are treating of have all some weight, however small ; 
and will, therefore, displace some quantity of water. If 
the body be lighter than water, it will not sink to a level 
with the surface of the water, and therefore it will not 
displace so much water as is equal to its bulk ; but it will 
displace as much as is equal to its weight. A ship, you 
must have observed, sinks to some depth in water, and 
the heavier it is laden the deeper it sinks, as it always 
displaces a quantity of water equal to its weight. 

Caroline. But you said just now, that in the immer- 
sion of gold, the bulk, and not the weight of body, was to 
be considered. 

Mrs. B. That is the case with all substances which 
are heavier than water ; but since those which are light- 
er do not displace so much as their own bulk, the quan- 
tity they displace is not a test of their specifick gravity. 

In order to obtain the specifick gravity of a body which 
is lighter than water, you must attach to it a heavy one, 
whose specifick gravity is known, and immerse them to 
gether ; the specifick gravity of the lighter body may then 
be easily calculated. 

Emily. But are there not some bodies which have ex- 
actly the same specifick gravity as water ? 

Mrs. B. Undoubtedly ; and such bodies will remain 
at rest in whatever situation they are placed in water. 



rent persons, that their mean specifick gravity was about l-9th less 
than common water. So long, therefore, astho lungs can be kept 
free from water, a person, although unacquainted with the art of 
swimming, will not completely sink. 



614. How loner will a person unacquainted with swimming 
remain in the water without sinking ? 615. How can the spe- 
cifick gravity of bodies lighter than water be obtained ? 616 

How will bodies of the same specifick gravity of water remain 
when immersed in it 3 






ON THE MECHANICAL PROPERTIES OF FLUIDS. 149 

Here is a piece of wood, which, by being impregnated 
with a little sand, is rendered precisely of the weight of 
an equal bulk of water ; in whatever part of this vessel of 
water you place it, you will find that it will remain sta- 
tionary. 

Caroline. I shall first put it at the bottom ; from 
thence, of course, it cannot rise, because it is not lighter 
than water. Now I shall place it in the middle of the 
vessel : it neither rises nor sinks, because it is neither 
tighter nor heavier than the water. Now I will lay it on 
the surface of the water ; but there it sinks a little — 
what is the reason of that, Mrs. B. 1 

Mrs. JB. Since it is not lighter than the water, it 
cannot float upon its surface ; since it is not heavier than 
water it cannot sink below its surface ; it will sink, there- 
fore, only till the upper surface of both bodies are on a 
level, so that the piece of wood is just covered with water. 
If you poured a few drops of water into the vessel, (so 
gently as not to increase their momentum by giving them 
velocity) they would mix with the water at the surface, 
and not sink lower. 

Caroline. This must, no doubt, be the reason why in 
drawing up a bucket of water out of a well, the bucket 
feels so much heavier when it rises above the surface of 
the water in the well ; for whilst you raise it in the wa- 
ter, the water within the bucket being of the same spe- 
cifick gravity as the water on the outside, will be wholly 
supported by the upward pressure of the water beneath 
the bucket, and consequently very little force will be re- 
quired to raise it ; but as soon as the bucket rises to the 
surface of the well you immediately perceive the increase 
of weight. 

Emily. And how do you ascertain the specifick gravity 
of fluids ? 

Mrs. B. By means of an instrument called an hy- 
drometer, which I will show you. It consists of a thin 
glass ball A, (fig. 8, plate XIII.) with a graduated tube 
B, and the specifick gravity of the liquid is estimated by 
the depth to which the instrument sinks in it. There is 

617. What solid body is of the same specifick gravity of water 

618. How is the specifick gravities of fluids ascertained ? 

619. How is a hydrometer constructed ?— - -620. Which figure 
represents an hydrometer ? 

1?* 



150 *>F SPRINGS, FOUNTAINS, &X. 

a smaller ball, C, attached to the instrument below, which 
contains a little mercury ; but this is merely for the pur- 
pose of equipoising the instrument, that it may remain 
upright in the liquid under trial. 

I must now take leave of you ; but there remain yet 
many observations to be made on fluids ; we shall, there- 
fore, resume this subject at our next interview. 



CONVERSATION XL 

OF SPRINGS, FOUNTAINS, &C 

Of the Ascent of Vapour and the Formation of Clouds ; 
Of the Formation and Fall of Rain, Spc. ; Of the 
Formation of Springs ; Of Rivers and Lakes ; Of 
Fountains. 

CAROLINE. 

There is a question I am very desirous of asking you 
respecting fluids, Mrs. B., which has often perplexed me. 
What is the reason that the great quantity of rain which 
falls upon the earth and sinks into it, does not, in the 
course of time, injure its solidity ? The sun and the wind 
I know, dry the surface, but they have no effect on the 
interiour parts, where there must be a prodigious accumu- 
lation of moisture. 

Mrs. B. Do you not know that, in the course of time, 
all the water which sinks into the ground rises out of it 
again ? It is the same water which successively forms 
seas, rivers, springs, clouds, rain, and sometimes hail, 
snow, and ice. If you will take the trouble of following 
it through these various changes, you will understand why 
the earth is not yet drowned by the quantity of water 
which has fallen upon it since its creation ; and you will 
even be convinced, that it does not contain a single drop 
more water now, than it did at that period. 

Let us consider how the clouds were originally formed. 
When the first rays of the sun warmed the surface of the 



621. What is the reason that the great quantity of rain which 
falls upon the earth and sinks into it. does not, in the course of 
time, injure* its solidi.y ? 



OF SPRINGS, FOUNTAINS, &C 151 

earth, the heat, by separating the particles of water, ren- 
dered them lighter than the air. This, you know, is the 
case with steam or vapour. What then ensues ? 

Caroline. When lighter than the air it will naturally 
rise ; and now I recollect your telling us in a preceding 
lesson, that the heat of the sun transformed the particles 
of water into vapour, in consequence of which it ascended 
into the atmosphere, where it formed clouds. 

Mrs. B. We have then already followed water through 
two of its transformations ; from water it becomes vapour, 
and from vapour clouds. 

Emily. But since this watery vapour is lighter than 
the air, why does it not continue to rise 1 and why does 
it unite again to form clouds 1 

Mrs. B. Because the atmosphere diminishes in den- 
sity, as it is more distant from the earth. The vapour 
therefore which the sun causes to exhale, not only from 
seas, rivers, and lakes, but likewise from the moisture on 
the land, rises till it reaches a region of air of its own spe- 
cifick gravity ; and there, you know, it will remain sta- 
tionary. By the frequent accession of fresh vapour it gra- 
dually accumulates, so as to form those large bodies of va- 
pour, which we call clouds ; and these at length becoming, 
too heavy for the air to support, fall to the ground. 

Caroline. They do fall to the ground, certainly, when 
it rains ; but according to your theory, I should have ima- 
gined, that when the clouds became too heavy for the 
region of air in which they were situated to support them, 
they would descend till they reached a stratum of air of 
their own weight, and not fall to the earth ; for as clouds 
are formed of vapour, they cannot be so heavy as the low- 
est regions of the atmosphere, otherwise the vapour would 
not have risen. 

Mrs. B. If you examine the manner in which the 
clouds descend, it will obviate this objection. In falling, 
several of the watery particles come within the sphere of 



62*2. What is the cause of the ascent of vapour or steam ? 

623. How are the clouds formed ? 624. But since vapour is 

lighter than the air, why does it not continue to rise ? and why 

does it unite again to form clouds ? 625. What prevents the 

clouds remaining in the atmosphere where they are formed ? 

626. Why do the clouds descend to the earth in drops of water 
instead of vapour, as they ascended ? 



152 OF SPRINGS, FOUNTAINS, &C. 

each other's attraction, and unite in the form of a drop of 
water. The vapour, thus transformed into a shower, is 
heavier than any part of the atmosphere, and consequent- 
ly descends to the earth. 

Caroline. How wonderfully curious ! 

Mrs. B. It is impossible to consider any part of na- 
ture attentively without being struck with admiration at 
the wisdom it displays ; and I hope you will never con- 
template these wonders without feeling your heart glow 
with admiration and gratitude towards their bounteous 
Author. Observe, that if the waters were never drawn 
out of the earth, all vegetation would be destroyed by the 
excess of moisture ; if, on the other hand, the plants 
were not nourished and refreshed by occasional showers, 
the drought would be equally fatal to them. If the 
clouds constantly remain in a state of vapour, they might, 
as you remarked, descend into a heavier stratum of the 
atmosphere, but could never fall to the ground ; or were 
the power of attraction more than sufficient to convert the 
vapour into drops, it would transform the cloud into a 
mass of water, which, instead of nourishing, would destroy 
the produce of the earth. 

Water then ascends in the form of vapour, and descends 
in that of rain, snow, or hail, all of which ultimately be- 
come water. Some of this falls into the various bodies 
of water on the surface of the globe, the remainder upon 
the land. Of the latter, part re-ascends in the form of 
vapour, part is absorbed by the roots of vegetables and 
part descends into the bowels of the earth, where it forms 
springs. 

Emily. Is rain and spring-water then the same ? 

Mrs. B. Yes, originally. The only difference be- 
tween rain and spring water, consists in the foreign par- 
ticles which the latter meets with and dissolves in its pas- 
sage through the various soils it traverses. 

Caroline. Yet spring water is more pleasant to the 
taste, appears more transparent, and, I should have sup- 
posed, would have been more pure than rain water. 

Mrs. B. No ; excepting distilled water, rain water is 
the most pure we can obtain ; and it is its purity which 
renders it insipid, whilst the various salts and different 

627. What are the several changes which water undergoes in 

its ascent and descent ? 628. What is the difference between 

rain and spring water ? 629. Which is the most pure ? 



OF SPRINGS, FOUNTAINS, &C 153 

ingredients, dissolved in spring water, give it a species of 
flavour, without in any degree affecting its transparency ; 
and the filtration it undergoes through gravel and sand in 
the bowels of the earth, cleanses it from all foreign matter 
which it has not the power of dissolving. 

When rain falls on the surface of the earth, it continues 
making its way downwards through the pores and cre-^ 
vices in the ground. When several drops meet in their 
subterraneous passage, they unite and form a little rivulet ; 
this, in its progress, meets with other rivulets of a similar 
description, and they pursue their course together in the 
bowels of the earth, till they are stopped by some sub- 
stance which they cannot penetrate. 

Caroline. But you said that water could penetrate even 
the pores of gold, and they cannot meet with a substance 
more dense 7 

Mrs. B. But water penetrates the pores of gold only 
when under a strong compressive force, as in the Floren- 
tine experiment ; now in its passage towards the centre 
of the earth, it is acted upon by no other power than gra- 
vity, which is not sufficient to make it force its way even 
through a stratum of clay. This species of earth, though 
not remarkably dense, being of great tenacity, will not 
admit the particles of water to pass. When water en- 
counters any substance of this nature, therefore, its pro- 
gress is stopped, and the pressure of the accumulating 
waters forms a bed, or reservoir. This will be more clear- 
ly explained by fig. 9, plate XIII. which represents a sec- 
tion, or the interiour of a hill or mountain. A is a body 
of water such as I have described, which when filled up 
as high as B (by the continual accession of water it re- 
ceives from the ducts or rivulets a, a, a, a,) finds a pas- 
sage out of the cavity, and, impelled by gravity, it runs 
on, till it makes its way out of the ground at the side of 
the hill, and there forms a spring, C. 

Caroline. Gravity impels downward towards the cen- 
tre of the earth ; and the spring in this figure runs in a 
horizontal direction. 

630. What renders spring water more pleasant to the taste, if 
it is less pure than rain water ? 631. How are springs and ri- 
vulets at first formed ? 632. Through what species of earth 

will not water pass ? 633. Which figure represents the manner 

in which springs are formed ? 634. How would you explain 

this figure ? 



154 OF SPRINGS, FOUNTAINS, &LC. 

Mrs. B. Not entirely. There is some declivity from 
the reservoir to the spot where the water isues out of 
the ground ; and gravity, you know, will bring bodies 
down an inclined plane, as well as in a perpendicular di- 
rection. 

Caroline. But though the spring may descend on first 
issuing, it must afterward rise to reach the surface of the 
earth ; and that is in direct opposition to gravity. 

Mrs. B. A spring can never rise above the level of the 
reservoir whence it issues ; it must, therefore, find a pas- 
sage to some part of the surface of the earth that is lower 
or nearer the centre than the reservoir. It is true that, 
in this figure, the spring rises in its passage from B to C 
occasionally ; but this, I think, with a little reflection, you 
will be able to account for. 

Emily. Oh yes ; it is owing to the pressure of fluids up- 
wards, and the water rises in the duct upon the same prin- 
ciple as it rises in the spout of a tea-pot ; that is to say, 
in order to preserve an equilibrium with the water in the 
reservoir. Now I think I understand the nature of 
springs ; the water will flow through a duct, whether as- 
cending or descending, provided it never rises higher than 
the reservoir. 

Mrs. B. Water may thus be conveyed to every part 
of a town, and to the upper part of the houses, if it is ori- 
ginally brought from a height superiour to any to which it 
is conveyed. Have you never observed, when the pave- 
ment of the streets have been mending, the pipes which 
serve as ducts for the conveyance of the water through 
the town 1 

Emily. Yes, frequently ; and I have remarked that 
when any of these pipes have been opened, the water 
rushes upwards from them with great velocity, which I 
suppose proceeds from the pressure of the water in the re- 
servoir, which forces it out. 

Caroline. I recollect having once seen a very curious 
glass, called Tantalus's cup; it consists of a goblet, con- 
taining a small figure of a man, and whatever quantity of 
water you pour into the goblet, it never rises higher than 

635. How high may a spring rise ? 636. On what princi- 
ple does water ascend as well as descend in its course, as is often 
the case in being carried in ducts ? 637. What is called Tan- 
talus's cup ? 



OF SPRINGS, FOUNTAINS, &C 155 

the breast of the figure. Do you know how that is con- 
trived 1 

Mrs. B. It is by means of a syphon, or bent tube, 
which is concealed in the body of the figure. It rises 
through one of the legs, as high as the breast, and there 
turning, descends through the other leg, and from thence 
through the foot of the goblet, where the water runs out. 
(fig. 1, plate XIV.) When you pour water into the glass 
A, it must rise in the syphon B, in proportion as it rises 
in the glass ; and when the glass is filled to a level with 
the upper part of the syphon, the water will run out 
through the other leg of the figure, and will continue run- 
ning out, as fast as you pour it in ; therefore the glass 
can never fill any higher. 

Emily. I think the new well that has been made at 
our country-house, must be of that nature. We had a 
great scarcity of water, and my father has been at con- 
siderable expense to dig a well ; after penetrating to a 
great depth before water could be found, a spring was at 
length discovered, but the water rose only a few feet above 
the bottom of the well ; and sometimes it is quite dry. 

Mrs. B. This has however, no analogy to Tantalus's 
cup, but is owing to the very elevated situation of your 
country house. 

Emily. I believe I guess the reason. There cannot 
be a reservoir of water near the summit of a hill ; as in 
such a situation there will not be a sufficient number of 
rivulets formed to supply one ; and without a reservoir, 
there can be no spring. In such situations, therefore, it 
is necessary to dig very deep, in order to meet with a 
spring ; and when we give it vent, it can rise only as 
high as the reservoir from whence it flows, which will be 
but little, as the reservoir must be situated at some con- 
siderable depth below the summit of the hill. 

Caroline. Your explanation appears very clear and 
satisfactory. But I can contradict it from experience. 
At the very top of a hill, near our country-house, there 
is a large pond, and, according to your theory, it would 
be impossible there should be springs in such a situation to 
supply it with water. Then you know that I have crossed 

638. By what means is the water prevented from rising to the 

head of the figure ? 639. Why must weJls on high land be dug 

deep in order to be supplied with water ? 



156 OF SPRINGS, FOUNTAINS, &C 

the Alps, and I can assure you, that there is a fine lake 
on the summit of Mount Cenis, the highest mountain we 
passed over. 

Mrs. B. Were there a lake on the summit of Mount 
Blanc, which is the highest of the Alps, it would indeed 
be wonderful. But that on Mount Cenis is not at all 
contradictory to our theory of springs ; for this mountain 
is surrounded by others much more elevated, and the 
springs which feed the lake must descend from reservoirs 
of water formed in those mountains. This must also be 
the case with the pond on the top of the hill ; there is 
doubtless some more considerable hill in the neighbour- 
hood which supplies it with water. 

Emily, I comprehend perfectly, why the water in our 
well never rises high : but I do not understand why it 
should occasionally be dry. 

Mrs. B. Because the reservoir from which it flows 
being in an elevated situation, is but scantily supplied 
with water ; after a long drought, therefore, it may be 
drained, and the spring dry, till the reservoir be reple- 
nished by fresh rains. It is not uncommon to see springs 
flow with great violence in wet weather, and at other 
times be perfectly dry. 

Caroline. But there is a spring in our grounds which 
more frequently flows in dry than in wet weather : how is 
that to be accounted for ? 

Mrs. B. The spring probably comes from a reservoir 
at a great distance, and situated very deep in the ground : 
it is, therefore, some length of time before the rain reaches 
the reservoir, and another considerable portion must 
elapse, whilst the water is making its way from the 
reservoir to the surface of the earth ; so that the dry wea- 
ther may probably have succeeded the rains before the 
spring begins to flow, and the reservoir may be exhausted 
by the time the wet weather sets in again. 

Caroline. I doubt not but this is the case, as the 
spring is in a very low situation, therefore the reservoir 
may be at a great distance from it. 

Mrs. B. Springs, which do not constantly flow are 
called intermitting, and are occasioned by the reservoir 

640. How clm the lake on Mount Cenis, one of the Alps, be 

reconciled to the theory of springs which has been given ? 

641. Why are wells frequently dry? 642. Why do somr 

springs flow more in dry than wet weather ? 643. What 

springs are called intermitting. 3 



i 



OF SPRINGS, FOUNTAINS, &C. 157 

being imperfectly supplied. Independently of the situ- 
ation, this is always the case when the duct or ducts 
which convey the water into the reservoir are smaller 
than those which carry it off. 

Caroline. If it run out faster than it run in, it will 
of course sometimes be empty. And do not rivers also 
derive their source from springs ? 

? Mrs. B. Yes, they generally take their source in 
mountainous countries, where springs are most abundant. 
Caroline. I understood you that springs were more 
rare in elevated situations. 

Mrs. B. You do not consider that mountainous coun- 
tries abound equally with high and low situations. Re- 
servoirs of water, which are formed in the bosom of moun- 
tains, generally find a vent either on their declivity, or in 
the valley beneath ; while subterraneous reservoirs formed 
in a plain, can seldom find a passage to the surface of the 

1 earth, but remain concealed, unless discovered by digging 

• a well. When a spring once issues at the surface of the 
earth it continues its course externally, seeking always a 

I lower ground, for it can no longer rise. 

Emily. Tuen what is the consequence, if the spring, 
or I should now rather call it a rivulet, runs into a situa- 
tion which is surrounded by higher ground ? 

Mrs. B. Its course is stopped, the water accumulates, 
and it forms a pool, pond, or lake, according to the di- 

i mensions of the body of water. The lake of Geneva, in 
all probability, owes it origin to the Rhone, which passes 
through it ; if, when this river first entered the valley, 
which now forms the bed of the Lake, it found itself sur- 
rounded by higher grounds, its waters would there accu- 

, mulate, till they rose to a level with that part of the valley 

j where the Rhone now continues its course beyond the 
Lake, and from whence it flows through valleys, occasion- 
ally forming other small lakes till it reaches the sea. 
Emily. And are not fountains of the nature of springs ? 
Mrs. B. Exactly. A fountain is conducted perpen- 
dicularly upwards, by the spout or adjutage A, through 

644. Why do rivers usually have their source in mountainous 
regions ? — : — 645. When a spring once issues from the surface of 

I the earth what is its course ? -646. What is the consequence 

! if a spring runs into a situation which is surrounded by higher 

i ground ? 647. How was lake Geneva probably formed ? 

648. Are artificial fountains of the nature of springs 
14 



a58 on the mechanical properties ov air. 

which it flows ; and it will rise nearly as high as the reser- 
voir B, from whence it proceeds. (Plate XIV. figure 2.) 

Caroline. Why not quite as high ? 

Mrs. B. Because it meets with resistance from the 
air in its ascent ; and its motion is impeded by friction 
against the spout, where it rushes out. 

Emily. But if the tube through which the water rises 
be smooth, can there be any friction ? especially with a 
fluid whose particles yield to the slightest impression. 

Mrs. B. Friction v as we observed in a former les- 
son,) may be diminished by polishing, but can never be 
entirely destroyed; and though fluids are less susceptible 
of friction than solid bodies, they are still affected by it. 
Another reason why a fountain will not rise so high as its 
reservoir, is, that as all the particles of water spout from 
the tube with an equal velocity, and as the pressure of the 
air upon the exteriour particles must diminish their velo- 
city, they will, in some degree, strike against the under 
parts, and force them sideways, spreading the column into 
a head, and rendering it both wider and shorter than it 
otherwise would be. 

At our next meeting, we shall examine the mechanical 
properties of the air, which, being an elastick fluid, differs 
in many respects from liquids. 

649. Which figure represents an artificial fountain ? 650. 

Why in that representation does not the water rise as high as the 
reservoir ? 



CONVERSATION XII. 

ON THE MECHANICAL PROPERTIES OF AIR. 

Of the Spring or Elasticity of the Air ; Of the weight 
of the Air ; Experiments with the Air Pump ; Of the 
Barometer ; Mode of weighing Air ; Spccifick Gravity 
of Air ; Of Pumps ; Description of the Sucking 
Pump; Description of the Forcing Pump. 

MRS. B. 

At our last meeting we examined the properties of 
fluids in general, and more particularly of such fluids as 
are called liquids. 



ON THE MECHANICAL PROPERTIES OF AIR. 159 

There is another class of fluids, distinguished by the 
name of aeriform or elastick fluids, the principal of which 
is the air we breathe, which surrounds the earth, and is 
called the atmosphere. 

Emily. There are then other kinds of air, besides the 
atmosphere. 

Mrs. B. Yes ; a great variety ; but they differ only 
in their chemical, and not in their mechanical properties ; 
and as it is the latter we are to examine, we shall not at 
present inquire into their composition, but confine our 
attention to the mechanical properties of elastick fluids in 
general. 

Caroline. And from whence arises this difference ? 

Mrs. B. There is no attraction of cohesion between 
the particles of elastick fluids ; so that the expansive pow- 
er of heat has no adversary to contend with but gravity ; 
any increase of temperature, therefore, expands elastick 
fluids prodigiously, and a diminution proportionally con- 
denses them. 

The most essential point in which air differs from other 
fluids, is by its spring or elasticity ; that is to say, its 
power of increasing or diminishing in bulk, according as 
it is more or less compressed ; a power of which I have 
informed you liquids are almost wholly deprived. 

Emily. I think I understand the elasticity of the air 

very well from what you formerly said of it ; (see ;d. 32.) 

but what perplexes me is, its having gravity ; if it is heavy 

and we are surrounded by it, why do we not feel its 

' weight ? 

Caroline. It must be impossible to be sensible of the 
weight of such infinitely small particles, as those of which 
the air is composed : particles which are too small to be 
seen, must be too light to be felt. 

Mrs. B. You are mistaken, my dear ; the air is much 
heavier than you imagine ; it is true, that the particles 
which compose it are small ; but then, reflect on their 
quantity : the atmosphere extends to about the distance 

651^ How are the fluids called air distinguished from liquids ? 

652. How do the other kinds of air differ from atmospherick 

air ? 653. Has the attraction of cohesion any influence upon 

the particles of elastick fluids ? 654. What effect does heat 

have on them ? 655. What is to be understood by the elasti- 
city of the atmosphere ? 656. To what distance from the earth 

does the atmosphere extend ? 






160 ON THE MECHANICAL PROPERTIES OP AIR. 

of 45 miles from the earth ; and its gravity is such, that 
a man of middling stature is computed (when the air is 
heaviest) to sustain the weight of about 14 tons.* 

Caroline. Is it possible ! I should have thought such 
a weight would have crushed any one to atoms. 

Mrs. B. That would, indeed, be the case, if it were 
not for the equality of the pressure on every part of the 
body ; but when thus diffused we can bear even a much 
greater weight, without any considerable inconvenience. 
In bathing we support the weight and pressure of the wa- 
ter, in addition to that of the atmosphere ; but because this 
pressure is equally distributed over the body, we are 
scarcely sensible of it ; whilst if your shoulders, your head, 
or any particular part of your frame were loaded with the 
additional weight of a hundred pounds, you would soon 
sink under the fatigue. Besides this, our bodies contain 
air, the spring of which counterbalances the weight of ex- 
ternal air, and renders us less sensible of its pressure. 

Caroline. But if it were possible to relieve me from the 
weight of the atmosphere, should I not feel more light 
and agile ? 

Mrs. B. On the contrary, t?he air within you, meeting 
with no external pressure to restrain its elasticity, would 
distend your body, and at length, bursting the parts which 
confined it, put a period to your existence. 

Caroline. This weight of the atmosphere, then, which 
I was so apprehensive would crush me, is, in reality, es- 
sential to my preservation. 

Emily. I once saw a person cupped, and was told 
that the swelling of the part under the cup was produced 
by taking away from that part the pressure of the atmo- 
sphere ; but I could not understand how this pressure pro- 
duced such an effect. 

Mrs. B. The air pump affords us the means of mak- 
ing a great variety of interesting experiments on the 
weight and pressure of the air : some of them you have 






* The height to which the atmosphere extends has never been 
accurately ascertained ; but at a greater distance than 45 miles it 
ceases to reflect the sun's rays. 

657. What weight of air is a common sized man supposed to 

sustain ? 658. Why does not such a weight crush him to 

atoms ? 659. What would bo the consequence, if the weight 

of cxtcrnril air were removed from us ? 



ON THE MECHANICAL PROPERTIES OF AIR. 161 

already seen. Do you not secollect, that in a vacuum pro- 
duced within the air pump, substances of various weights 
fell to the bottom in the same time ? why does not this 
happen in the atmosphere 1 

Caroline, I remember you told us it was owing to the 
resistance which light bodies meet with from the air dur- 
ing their fall. 

Mrs. B. Or, in other words, to the support which 
they received from the air, and which prolonged the time 
of their fail. Now, if the air were destitute of weight, 
how could it support other bodies or retard their fall 1 

I shall now show you some other experiments, which 
illustrate, in a striking manner, both the weight and elas- 
ticity of air. I shall tie a piece of bladder over this glass 
receiver, which, you will observe, is open both at the top 
as well as below. 

Caroline. Why do you wet the bladder first 1 

Mrs. B. It expands by wetting, and contracts in 
drying ; it is also more soft and pliable when wet, so 
that I can make it fit better, and when dry it will be 
tighter. We must hold it to the fire in order to dry ; 
but not too near, lest it should burst by sudden contrac- 
tion. Let us now fix it on the air-pump and exhaust the 
air from underneath it — you will not be alarmed if you 
hear a noise. 

Emily. It was as loud as the report of a gun, and the 
bladder is burst ! Pray explain how the air is concerned 
in this experiment. 

Mrs. B. It is the effect of the weight of the atmo- 
sphere on the upper surface of the bladder, when I had ta- 
ken away the air from the under surface ; so that there 
was no longer any re-action to counterbalance the pres- 
sure of the atmosphere on the receiver. You observed 
how the bladder was pressed inwards by the weight of 
the external air, in proportion as I exhausted the receiver : 
and before a complete vacuum was formed, the bladder, 

660. Why do not bodies of various weights in the atmosphere 

fall in the same time ? 661. What does the fact prove, that 

light bodies are retarded by the air in falling to the earth ?— 662. 
How may it be shown that the air has weight ? 

14 * 



1G2 ON THE MECHANICAL PROPERTIES OP AIR. 

unable to sustain the violence of the pressure, burst with 
the explosion you have just heard.* 

I shall now show you an experiment, which proves the 
expansion of the air, contained within a body when it is 
relieved from the pressure of the external air. You 
would not imagine that there was any air contained with- 
in this shrivelled apple, by its appearance ; but take no- 
tice of it when placed within a receiver, from which I 
shall exhaust the air. 

Caroline. How strange ! it grows quite plump, and 
looks like a fresh-gathered apple. 

Mrs. B. But as soon as I let the air again into the 
receiver, the apple you see returns to its shrivelled state. 
When I took away the pressure of the atmosphere, the air 
within the apple expanded and swelled it out ; but the 
instant the atmospherical air was restored, the expansion 
of the internal air was checked and repressed, and the 
apple shrunk to its former dimensions. 

You may make a similar experiment with this little 
bladder, which you see is perfectly flaccid and appears to 
contain no air : in this state I shall tie up the neck of 
the bladder, so that whatever air remains within it may 
not escape, and then place it under the receiver. Now 
observe, as I exhaust the receiver, how the bladder dis- 
tends ; this proceeds from the great dilatation of the 
small quantity of air which was enclosed within the blad- 
der when I tied it up: but as soon as I let the air into 
the receiver, that which the bladder contains, condenses 



* The weight of the atmosphere can also be ascertained from 
the following experiments. — The air being exhausted, by an air- 
pump, from a glass receiver, the receiver will be held fast by the 
pressure of the external air. If a small receiver be placed under 
a larger one, and the air be exhausted from both, the larger one 
will be held fast by the pressure of external air, while the smaller 
one will be easily moved. Or, if the hand be placed upon a small 
open vessel in such a manner as to close its upper orifice, it wilj be 
held down with great force. 

663. What experiments named in the note prove that air has 

weight f 664. How may the elasticity or expansive power of 

the air be shown ? 



ON THE MECHANICAL PROPERTIES OP AIR. 163 

and shrinks into its small compass within the folds of the 
bladder.* 

Emily. These experiments are extremely amusing, 
and they afford clear proofs both of the weight and elas- 
ticity of the air ; but I should like to know exactly how 
much the air weighs. 

Mrs. B. A column of air reaching to the top of the 
atmosphere, and whose base is a square inch, weighs 151bs. 
when the air is heaviest ; therefore every square inch of 
our bodies sustains a weight of 151bs. : and if you wish to 
know the weight of the whole of the atmosphere, you must 
reckon how many square inches there are on the surface 
of the globe, and multiply them by 15.f 

Emily. But are there no means of ascertaining the 
weight of a small quantity of air ? 

Mrs. B. Nothing more easy. I shall exhaust the air 
from this little bottle by means of the air pump : and hav- 
ing emptied the bottle of air, or, in other words, pro- 
duced a vacuum within it, I secure it by turning this screw 
adapted to its neck : we may now find the exact weight 
of this bottle, by putting it into one of the scales of a ba- 
lance. It weighs you see just two ounces ; but when I 
turn the screw, so as to admit the air into the bottle, the 
scale which contains it preponderates. 

Caroline. No doubt, the bottle filled with air, is hea- 
vier than the bottle void of air ; and the additional weight 
required to bring the scales again to a balance, must be 
exactly that of the air which the bottle now contains. 

* If a tube, closed at one end, be inserted at its open end, in a 
vessel of water, the fluid in the tube will not rise to the level of the 
water in the vessel, being resisted by the elastick force of the air 
within the tube. It is on this principle that the diving bell is 
formed. 



t It has been computed that the pressure of the atmosphere on 
the whole surface of the earth is equivalent to that of a globe of 
lead sixty miles in diameter. 

665. How much does a column of air, reaching to the top of the 

atmosphere, of an inch in diameter, weigh ? 666. How great 

has been estimated the whole pressure of the atmosphere upon the 

earth f 667. How can the weight of a small quantity of air be- 

ascertained ? 



164 ON THE MECHANICAL PROPERTIES OF AIR. 

Mrs. B. That weight, you see, is almost two grains. 
The dimensions of this bottle are six cubick inches. Six 
cubick inches of air, therefore, at the temperature of this 
room, weigh nearly two grains. 

Caroline. Why do you observe the temperature of 
the room in estimating the weight of the air ? 

Mrs. B. Because heat rarefies air, and renders it 
lighter ; therefore the warmer the air is which you weigh, 
the lighter it will be. 

If you should now be desirous of knowing the spe- 
cifick gravity of this air, we need only fill the same bottle 
with water, and thus obtain the weight of an equal quan- 
tity of water — which you see is 1515 grains ; now by 
comparing the weight of water to that of air we find it to 
be in the proportion of about 800 to 1. 

I will show you another instance of the weight of the 
atmosphere, which T think will please you : you know 
what a barometer is 1 

Caroline. It is an instrument which indicates the state 
of the weather, by means of a tube of quicksilver ; but 
how, I cannot exactly say. 

Mrs. B. It is by showing the weight of the atmo- 
sphere. The barometer is an instrument extremely simple 
in its construction : in order that you may understand it, 
I will show you how it is made. I first fill a glass tube A 
B, (fig .3, plate XIV.) abouUthree feet in length, and open 
only at one end, with mercury ; then stopping the open 
end with my finger, I immerse it in a cup C, containing 
a little mercury. 

Emily. Part of the mercury which was in the tube, 
I observe, runs down into the cup ; but why does not the 
whole of it subside in the cup, for it is contrary to the law 
of the equilibrium of fluids, that the mercury in the tube 
should not descend to a level with that in the cup. 

Mrs. B. The mercury that has fallen from the tube 
into the cup, has left a vacant space in the upper part of 
the tube, to which the air cannot gain access ; this space 
is therefore a perfect vacuum ; and consequently the 

668. Why is it necessary in this experiment to observe the 

temperature of the room in which it is made ? 669. How much 

heavier is water than air ? 670. How is the specifick gravity 

of air determined ? 671. What is a barometer ? 672. Which 

figure represents a barometer ? 673. How is the weight of the 

atmosphere determined by a barometer ? 



ON THE MECHANICAL PROPERTIES OF AIR. 165 

mercury in the tube is relieved from the pressure of the 
atmosphere, whilst that in the cup remains exposed to it. 

Caroline. Oh, now I understand it ; the pressure of 
the air on the mercury in the cup forces it to rise in the 
tube, where it sustains no pressure. 

Emily. Or rather supports the mercury in the tube, 
and prevents it from falling. 

Mrs. B. That comes to the same thing ; for the pow- 
er that can support mercury in a vacuum, would also make 
it ascend when it met with a vacuum. 

Thus you see, that the equilibrium of the mercury is 
destroyed only to preserve the general equilibrium of 
fluids. 

Caroline. But this simple apparatus is, in appearance, 
very unlike a barometer. 

Airs. B. It is all that is essential to a barometer 
The tube and the cup or vase are fixed on a board, for 
the convenience of suspending it ; the board is graduated 
for the purpose of ascertaining the height at which the 
mercury stands in the tube ; and the small moveable me- 
tal plate serves to show that height with greater accuracy. 

Emily. And at what height will the weight of the at- 
mosphere sustain the mercury 1 

Mrs. B. About 28 inches, as you will see by this 
barometer ; but it depends upon the weight of the atmo- 
sphere, which varies much according to the state of the 
weather. The greater the pressure of the air on the mer- 
cury in the cup, the higher it will ascend in the tube, 
Now can you tell me whether the air is heavier in wet or 
dry weather 1 

Caroline. Without a moment's reflection, the air 
must be heaviest in wet weather. It is so depressing, and 
makes one feel so heavy ; while in fine weather, I feel as 
light as a feather, and as brisk as a bee. 

Mrs. B. Would it not have been better to have an- 
swered with a moment's reflection, Caroline ? It would 
have convinced you, that the air must be heaviest in dry 
weather, for it is then, that the mercury is found to rise 
in the tube, and consequently the mercury in the cup 

674. At what height will the weight of the atmosphere sustain 

the mercury ? 675. According to what does the weight of the 

atmosphere vary ? 676. When is the ?,ir the heaviest, in wet 

or drv weather ? 



100 ON THE MECHANICAL PROPERTIES OP AIR. 

must be most pressed by the air : and you know, that we 
estimate the dryness and fairness of the weather, by the 
height of the mercury in the barometer. 

Caroline. Why then does the air feel so heavy in bad 
weather ? 

Mrs. B. Because it is less salubrious when impreg 
nated with damp. The lungs under these circumstances 
do not play so freely, nor does the blood circulate so well : 
thus obstructions are frequently occasioned in the smaller 
vessels, from which arise colds, asthmas, agues, fevers, 
&,c. 

Emily. Since the atmosphere diminishes in density in 
the upper regions, is not the air more rare upon a hill 
than in a plain ; and does the barometer indicate this 
difference ? 

Mrs. B. Certainly. The hills in this country are not 
sufficiently elevated to produce any very considerable ef- 
fect on the barometer ; but this instrument is so exact in 
its indications, that it is used for the purpose of measuring 
the height of mountains, and of estimating the elevation 
of balloons. 

Emily. And is no inconvenience experienced from 
the thinness of the air in such elevated situations ? 

Mrs. B. Oh, yes ; frequently. It is sometimes op- 
pressive, from being insufficient for respiration ; and the 
expansion which takes place in the more dense air con- 
tained within the body is often painful : it occasions dis- 
tension, and sometimes causes the bursting of the smaller 
blood-vessels in the nose and ears. Besides, in such situ- 
ations, you are more exposed both to heat and cold ; for 
though the atmosphere is itself transparent, its lower re- 
gions abound with vapours and exhalations from the earth, 
which float in it, and, act in some degree as a covering, 
which preserves us equally from the intensity of the sun's 
rays, and from the severity of the cold. 

Caroline. Pray, Mrs. B., is not the thermometer con- 
structed on the same principles as the barometer 1 

Mrs. B. Not at all. The rise and fall of the fluid 
in the thermometer is occasioned by the expansive power 

677. Why then do our feelings indicate that the air is heaviest 
in wet weather, if that is not the fact ? 678. Is the atmo- 
sphere of the same density on a hill or mountain as in a valley ? 
679. Does a person in elevated situations feel any inconveni- 
ence from the thinness of the atmosphere ? 680. What causes 

the rise and fall of the fluid in the thermometer ? 



ON THE MECHANICAL PROPERTIES OF AIR. 167 

of heat, and the condensation produced by cold ; the air has 
no access to it. An explanation of it would, therefore, 
be irrelevant to our present subject. 

Emily, I have been reflecting, that since it is the 
weight of the atmosphere which supports the mercury in 
the tube of a barometer, it would support a column of any 
other fluid in the same manner. 

Mrs. B. Certainly ; but as mercury is heavier than 
all other fluids, it will support a higher column of any 
other fluid ; for two fluids are in equilibrium, when their 
height varies inversely as their densities. We find the 
weight of the atmosphere is equal to sustaining a column 
of water, for instance, of no less than 32 feet above its 
level. 

Caroline. The weight of the atmosphere is, then, as 
great as that of a body of water the depth of 32 feet ? 
- Mrs. B. Precisely ; for a column of air of the height 
of the atmosphere, is equal to a column of water of 32 
feet, or one of mercury of 28 inches. 

The common pump is constructed on this principle. By 
the act of pumping, the pressure of the atmosphere is ta- 
ken off the water, which, in consequence, rises. 

The body of a pump consists of a large tube or pipe, 
whose lower end is immersed in the water which it is de- 
signed to raise. A kind of stopper, called a piston, is fit- 
ted to this tube, and is made to slide up and down it by 
means of a metallick rod fastened to the centre of the pis- 
ton. 

Emily. Is it not similar to the syringe, or squirt, with 
which you first draw in, and then force out water ? 

Mrs. B. It is ; but you know that we do not wish to 
force the water out of the pump at the same end of the 
oipe at which we draw it in. The intention of a pump 
is to raise water from a spring of well ; the pipe is there- 
fore placed perpendicularly over the water which enters 
it at the lower extremity, and it issues at a horizontal 
spout towards the upper part of the pump. The pump 

681. Will the weight of the atmosphere support other fluids 

than mercury ? -682. What fluid is heaviest ? 683. When 

are two fluids of different density in equilibrium ? 684. How 

high a column of water wilPthe weight of the atmosphere sustain ? 
— ■ — 685. What instrument in common use is constructed on this 

principle ? 686. What causes the water to rise in a pump ? 

687. How is a common pump constructed ? 



168 ON THE MECHANICAL PROPERTIES OF AIR. 

therefore, is rather a more complicated piece of machine- 
ry than the syringe. 

Its various parts are delineated in this figure : (fig. 4. 
plate XIV.) A B is the pipe or body of the pump, P the 
piston, V a valve, or little door in the piston, which open- 
ing upwards, admits the water to rise through it, but pre- 
vents its returning, and Y a similar valve in the body of 
the pump. 

When the pump is in a state of inaction, the two valves 
are closed by their own weight ; but when, by drawing 
down the handle of the pump, the piston ascends, it raises 
a column of air which rested upon it, and produces a va- 
cuum between the piston and the lower valve Y, the air 
beneath this valve, which is immediately over the surface 
of the water, consequently expands, and forces its way 
through it ; the water, then, relieved from the pressure 
of the air, ascends into the pump. A few strokes of the- 
handle totally excludes the air from the body of the pump, 
and fills it with water, which, having passed through both 
the valves, runs out at the spout. 

Caroline. I understand this perfectly. When the 
piston is elevated, the air and the water successively rise 
in the pump ; for the same reason as the mercury rises in 
the barometer. 

Emily. I thought that water was drawn up into a 
pump, by suction, in the same manner as water may be 
sucked through a straw. 

Mrs. B. It is so, into the body of the pump ; for the 
power of suction is no other than that of producing a va- 
cuum over one part of the liquid, into which vacuum the 
liquid is forced, by the pressure of the atmosphere on 
another part. The action of sucking through a straw, 
consists in drawing in and confining the breath, so as to 
produce a vacuum in the mouth ; in consequence of which 
the air within the straw rushes into the mouth, and is fol- 
lowed by the liquid, into which the lower end of the 
straw is immersed. The principle, you see, is the same, 
and the only difference consists in the mode of producing 



688. How would you explain the pump, by reference to fi>. 4. 

plate XIV. ? 689. Is the power of suction, and that which 

s water to rise in a pump, the same ? 690. What is the 

power of taction f 691. In what consists the action of sucking 

liquid through a straw or any small tube r 



ON THE MECHANICAL PROPERTIES OF AIR. 169 

a vacuum. In suction, the muscular powers answer the 
purpose of the piston and valves. 

Emily. Water cannot, then, be raised by a pump 
above 32 feet ; for the pressure of the atmosphere will 
not sustain a column of w T ater above that height. 

Mrs. B. I beg your pardon. It is true that there 
must never be so great a distance as 32 feet from the 
level of the water in the well, to the valve in the piston, 
otherwise the water would not rise through that valve ; 
but when once the water has passed that opening, it is no 
longer the pressure of air on the reservoir which makes it 
ascend ; it is raised by lifting it up, as you would raise 
it in a bucket, of which the piston formed the bottom. 
This common pump is, therefore, called the sucking, or 
lifting-pump, as it is constructed on both these principles. 
There is another sort of pump, called the forcing-pump . 
it consists of a forcing power added to the sucking part of 
the pump. This additional power is exactly on the prin- 
ciple of the syringe : by raising the piston you draw the 
water into the pump, and by descending it you force the 
water out. 

Caroline. But the water must be forced out at the 
upper part of the pump ; and I cannot conceive how that 
can be done by descending the piston. 

Mrs. B. Figure 5, plate XIV. will explain the diffi- 
culty. The large pipe A B represents the sucking part 
of the pump, which differs from the lifting-pump, only in 
its piston P being unfurnished with a valve, in consequence 
of which the water cannot rise above it. When, there- 
fore, the piston descends, it shuts the valve Y, and forces 
the water (which has no other vent) into the pipe D : this 
is likewise furnished with a valve V, which, opening out- 
wards, admits the water, but prevents its return. 

The water is thus first raised in the pump, and then 
forced into the pipe, by the alternate ascending and de- 
scending motion of the piston, after a few strokes of the 



692. What in suction answer the purpose of the piston and 

valves of the pump ? 693. Can water be raised in a pump 

more than 32 feet ? 694. How can it, if the weight of the at- 
mosphere is only equal to a column of water of that height ? 

695. Of what does the forcing pump consist ? 696. Which 

figure represents the forcing pump ? 697. How would you ex- 
plain the forcing pump by the figure ? 
15 



nO ON WIND AND SOUND 

handle to fill the pipe, from whence the water issues at 
the spout. 

It is now time to conclude our lesson. When next we 
meet, I shall give you some account of wind, and of 
sound, which will terminate our observations on elastick 
fluids. 

Caroline. And I shall run into the garden, to have 
the pleasure of pumping, now that I understand the con- 
struction of a pump. 

Mrs. B. And, to-morrow I hope you will be able to 
tell me, whether it is a forcing or a common lifting pump. 



CONVERSATION XIII. 

ON WIND AND SOUND. 

Of Wind in General; Of the Trade Wind; Of the 
Periodical Trade Winds ; Of the Aerial Tides ; Of 
Sounds in General ; Of Sonorous Bodies ; OfMusica* 
Sounds; Of Concord or Harmony ', and Melody. 

MRS. B. 

Well, Caroline, have you ascertained what kind of 
' pump you have in your garden ? 

Caroline. I think it must be merely a lifting-pump, 
because no more force is required to raise the handle than 
is necessary to lift its weight ; and in a forcing pump, by 
raising the handle, you force the water into the smaller 
pipe, and the resistance the water offers must require an 
exertion of strength to overcome it. 

Mrs. B. I make no doubt you are right ; for lifting 
pumps, being simple in their construction, are by far the 
most common. 

I have promised to-day to give you some account of 
the nature of wind. Wind is nothing more than the mo- 
tion of a stream or current of air, generally produced by 
a partial change of temperature in the atmosphere ; for 
when any one part is more heated than the rest, that part 
is rarefied ; the equilibrium is destroyed, and the air in 
consequence rises. When this happens, there neces- 

698. What is wind ? Cl>0. How is the air put in motion to 

as to produce wind ; 



ON WIND AND SOUND. 171 

sarily follows a motion of the surrounding air towards that 
part, in order to restore it ; this spot, therefore, receives 
winds from every quarter. Those who live to the north 
of it experience a north wind ; those to the south, a south 
wind :— do you comprehend this ?* 

Caroline. Perfectly. But what sort of weather must 
those people have who live on the spot where these winds 
meet and interfere ? 

Mrs. B. They have turbulent and boisterous wea- 
ther, whirlwinds, hurricanes, rain, lightning, thunder, &c. 
This stormy weather occurs most frequently in the torrid 
zone, where the heat is greatest : the air, being more ra- 
refied there than in any other part of the globe, is light- 
er, and consequently ascends ; whilst the air about the 
polar regions is continually flowing from the poles to re- 
store the equilibrium. 

Caroline. This motion of the air would produce a re- 
gular and constant north wind to the inhabitants of the 
northern hemisphere ; and a south wind to those of the 
southern hemisphere ; and continual storms at the equa- 
tor, where these two adverse winds would meet. 

Mrs. B. These winds do not meet, for they each 
change their direction before they reach the equator. 
The sun, in moving over the equatorial regions from east 
to west, rarefies the air as it passes, and causes the den- 
ser eastern air to flow westwards, in order to restore the 
equilibrium ; thus producing a regular east wind about 
the equator. 

Caroline. The air from the west, then, constantly 
goes to meet the sun, and repair the disturbance which 

* Fill a large dish with cold water ; into the middle of this put a 
waiter, filled with warm water. The first will represent the ocean 
and the other an island, rarefying the air above it. Blow out a wax 
candle, and if the air be still, on applying it successively to every 
side of the dish, the smoke will be seen to move towards the plate. 
—-Again, if the ambient water be warmed and the plate be filled 
with cold water, let the wick of smoking candles be held over the 
plate, and the contrary will happen. 

700. What illustration of wind produced by change of tempera- 
ture is given in the note ? 701. What is the consequence 

when winds from different quarters meet or interfere ? 702. 

Where does this mostly happen ? 703. Why does this mostly 

happen in the torrid zone ? 704. What regular wind prevails 

about the equator ? 705. Why is there a regular east wind at 

and near the equator D 



172 ON WIND AND SOUND. 

his beams have produced in the equilibrium of the atmo 
sphere. But I wonder how you will reconcile these va- 
rious winds, Mrs. B. ; you first led me to suppose there 
was a constant struggle between opposite winds at the 
equator producing storm and tempest ; but now I hear of 
one regular invariable wind, which must naturally be at- 
tended by calm weather. 

Emily, I think I comprehend it : do not these winds 
from the north and south combine with the easterly wind 
about the equator, and form what are called the trade- 
winds ? 

Mrs. B. Just so, my dear. The composition of the 
two winds north and east, produces a constant north-east 
wind ; and that of the two winds south and east, produces 
a regular south-east wind : these winds extend to about 
thirty degrees on each side of the equator, the regions fur- 
ther distant from it experiencing only their respective 
north and south winds.* 

Caroline. But, Mrs. B., if the air is constantly flow- 
ing from the poles to the torrid zone, there must be a de- 
ficiency of air in the polar regions ? 

Mrs. B. The light air about the equator, which ex- 
pands and rises into the upper regions of the atmosphere, 
ultimately flows from thence back to the poles, to restore 
the equilibrium : if it were not for this resource, the po- 
lar atmospherick regions would soon be exhausted by the - 
stream of air, which, in the lower strata of the atmosphere, 
they are constantly sending towards the equator. 

Caroline. There is then a sort of circulation of air in 
the atmosphere ; the air in the lower strata flowing from 
the poles towards the equator, and in the upper strata flow- 
ing back from the equator towards the poles. 

* On the coast of America, the trade winds are felt as far as forty 
degrees from the equator. By the aid of these winds, vessels sailing 
from Mexico to the Philippine islands, often finish a voyage, nearly 
equal to half the circumference of the globe, in CO days, without 
altering their course, or changing a sail. But in returning, they 
are obliged to go north, beyond the limits of the trade winds. 

704. How are the trade winds occasioned ? 705. How far 

on each side of the equator do these winds extend ?— — 706. What 

is said of the trade winds on the coast of America ? 707. What 

fact is mentioned of vessels sailing from Mexico to the Philippine 

islands f 708. Why do not the polar regions become exhausted 

of air, if it is continually blowing from them to the equator ? 



ON WIND AND SOUND. 173 

Mrs. B. Exactly. I can show you an example of this 
circulation on a small scale. The air of this room being 
more rarefied than the external air, a wind or current of 
air is pouring in from the crevices of the windows and 
doors, to restore the equilibrium ; but the light air with 
which the room is filled must find some vent, in order to 
make way for the heavy air which enters. If you set the 
door a-jar, and hold a candle near the upper part of it, you 
will find that the flame will be blown outwards, showing 
that there is a current of air flowing out from the upper 
part of the room. Now place the candle on the floor 
close by the door, and you will perceive, by the inclina- 
tion of the flame, that there is also a current of air setting 
into the room. 

Caroline. It is just so ; the upper current is the warm 
light air, which is driven out to make way for the stream 
of cold dense air which enters the room lower down. 

Emily. I have heard, Mrs. B., that the periodical 
winds are not so regular on land as at sea ; what is the 
reason of that 1 

Mrs. B. The land reflects into the atmosphere a 
much greater quantity of the sun's rays than the water ; 
therefore that part of the atmosphere which is over the 
land is more heated and rarefied than that which is over 
the sea : this occasions the wind to set in upon the land, 
as we find that it regularly does on the coast of Guinea, 
and other countries in the torrid zone. 

Emily. I have heard much of the violent tempests oc- 
casioned by the breaking up of the monsoons ; are not 
they also regular trade winds 1 

Mrs. B. They are called periodical trade-winds, as 
they change their course every half-year. This varia- 
tion is produced by the earth's annual course round the 
sun, when the north pole is inclined towards that lumina- 
ry one half of the year, the south pole the other half. Du- 
ring the summer of the northern hemisphere, the countries 
of Arabia, Persia, India, and China, are much heated, 



709. What familiar illustration can you give of the circulation 
of the air, first from the poles to the equator, and then rising and 

returning to the poles ? 710. Why are the periodical winds 

more regular at sea than on land ? 711. What winds are call- 
ed monsoons ? — -^712. How is the variation of the monsoons 
produced } 

15* 



174 ON WIND AND SOUND. 

and reflect great quantities of the sun's rays into the at 
mosphere, by which it becomes extremely rarefied, and 
the equilibrium consequently destroyed. In order to re- 
store it, the air from the equatorial southern regions, where 
it is colder, (as well as from the colder northern parts,) 
must necessarily have a motion towards those parts. The 
current of air from the equatorial regions produces the 
trade-winds for the first six months, in all the seas between 
the heated continent of Asia, and the equator. The other 
six months, when it is summer in the southern hemi- 
sphere, the ocean and countries towards the southern 
tropick are most heated, and the air over those parts most 
rarefied : then the air about the equator alters its course, 
and flows exactly in an opposite direction.* 

Caroline. This explanation of the monsoons is very 
curious ; but what does their breaking up mean ? 

Mrs. B. It is the name given by sailors to the shifting 
of the periodical winds ; they do not change their course 
suddenly, but by degrees, as the sun moves from one he- 
misphere to the other : this change is usually attended by 
storms and hurricanes, very dangerous for shipping ; so 
that those seas are seldom navigated at the season of the 
equinox. 

Emily. I think I understand the winds in the torrid 
zone perfectly well ; but what is it that occasions the 
great variety of winds which occur in the temperate zones ? 
for according to your theory, there should be only north 
and south winds in those climates. 

Mrs. B. Since so large a portion of the atmosphere as 
is over the torrid zone is in continued agitation, these agi- 
tations in an elastick fluid, which yields to the slightest 
impression, must extend every way to a great distance ; 
the air, therefore, in all climates, will suffer more or less 
perturbation, according to the situation of the country, 
the position of mountains, valleys, and a variety of other 
causes : hence it is easy to conceive, that almost every 
climate must be liable to variable winds. 

* The south-west monsoon, which blows from April to October, 
brings with it floods of rain, and dreadful tempests. During the 
rest of the year, the north-east monsoon produces a dry and agree- 
able state of the air. 

713. What effect do the monsoons have on the weather ? 

714. What does the breaking up of the monsoons mean ? 

715. What is it that occasions the great variety of winds which 
occur in the temperate zone? ? 



ON WIND AND SOUND. , 175 

On the sea-shore, there is almost always a gentle sea-breeze 
setting in on the land on a summer's evening, to restore the 
equilibrium which had been disturbed by reflections from 
the heated surface of the shore during the day ; and when 
night has cooled the land, and condensed the air, we ge- 
nerally find it, towards morning, flowing back towards the 
sea. 

Caroline. I have observed that the wind, whichever 
way it blows, almost always falls about sun-set. 

Mrs. B. Because the rarefaction of air in the particu- 
lar spot which produces the wind, diminishes as the sun 
declines, and consequently the velocity of the wind abates. 

Emily. Since the air is a gravitating fluid, is it not 
affected by the attraction of the moon and the sun, in the 
same manner as the waters 1 

Mrs. B. Undoubtedly ; but the aerial tides are as 
much greater than those of water, as the density of water 
exceeds that of air, which, as you may recollect, we found 
to be about 800 to 1. 

Caroline. What a prodigious protuberance that must 
occasion ; how much the weight of such a column of air 
must raise the mercury in the barometer ! 

Emily. As this enormous tide of air is drawn up and 
supported, as it were by the moon, its weight and pres- 
sure, I should suppose, would be rather diminished than 
increased ? 

Mrs. B. The weight of the atmosphere is neither in- 
creased nor diminished by the aerial tides. The moon's 
attraction augments the bulk as much as it diminishes 
the weight of the column of air ; these effects, therefore, 
counterbalancing each other, the aerial tides do not affect 
the barometer. 

Caroline. I do not quite understand that. 

Mrs. B. Let us suppose that the additional bulk of 
air at high tide raises the barometer one inch ; and on 
the other hand, that the support which the moon's attrac- 
tion affords the air, diminishes its weight or pressure, so 
as to occasion the mercury to fall one inch ; under these 



716. What are the sea-breezes as they are termed ? 717. 

Why does the wind generally subside at the going down of the sun? 

718. Does the moon have any effect on the wind ? 719. 

How much greater are the aerial tides than those of water ?— 
720. Why do not the aerial tides affect the barometer ? 



176 ON WIND AND SOUND. 

circumstances the mercury must remain stationary. Thus 
you see, that we can never be sensible of aerial tides by 
the barometer, on account of the equality of pressure of 
the atmosphere, whatever be its height. 

The existence of aerial tides is not, however, hypo- 
thetical ; it is proved by the effect they produce on the 
apparent position of the heavenly bodies ; but this I can- 
not explain to you, till you understand the properties of 
light.* 

Emily. And when shall we learn them ? 

Mrs. B. I shall first explain to you the nature of 
sound, which is intimately connected with that of air ; 
and I think at our next meeting we may enter upon the 
subject of opticks. 

We have now considered the effects produced by the 
wide and extended agitation of the air ; but there is ano- 
ther kind of agitation of which the air is susceptible — a 
sort of vibratory, trembling motion, which, striking on the 
drum of the ear, produces sound.f 

Caroline. Is not sound produced by solid bodies? 
The voice of animals, the ringing of bells, musical in- 
struments, are all solid bodies. I know of no sound but 
that of the wind which is produced by the air. 

Mrs. B. Sound, I assure you, results from a tremu- 
lous motion of the air ; and the sonorous bodies you enu- 
merate, are merely the instruments by which that peculiar 
species of motion is communicated to the air. 



* The quality of winds is affected by the countries over which 
they pass ; and they are sometimes rendered pestilential by the 
heat of deserts, or the putrid exhalations of marshes and lakes. 
Thus, from the deserts of Africa, Arabia, and the neighbouring 
countries, a hot wind blows, called Sarniel or Simoom, which some- 
times produces instant death. A similar wind blows from the Sa- 
hara, upon the western coast of Africa, called the Harmattan, pro- 
ducing a dryness and heat which is almost insupportable, and 
scorching like the blasts of a furnace. 

t The science which treats of the nature, phenomena, and laws 
of sound, is called Acoustichs. This science is particularly inte- 
resting and valuable from its extending to the theory of musical con- 
cord and harmony. 



721. By what is the quality ofwiuds affected ? 722. What 

facts are stated in the notes illustrating the effects thus produced 
on the wind ? 72H. How is sound produced ? 



ON WIND AND SOUND. 177 

Caroline. What ! when I ring this little bell, is it the 
air that sounds, and not the bell ? 

Mrs. B. Both the bell and the air are concerned in 
the production of sound. But sound, strictly speaking, 
is a perception excited in the mind by the motion of the 
air on the nerves of the ear ; the air, therefore, as well as 
the sonorous bodies which put it in motion, is only the 
cause of sound, the immediate effect is produced by the 
sense of hearing : for, without this sense, there would be 
no sound. 

Emily. I can with difficulty conceive that. A person 
born deaf, it is true, has no idea of sound, because he hears 
none ; yet that does not prevent the real existence of 
sound, as all those who are not deaf can testify. 

Mrs, B. I do not doubt the existence of sound to all 
those who possess the sense of hearing ; but it exists 
neither in the sonorous body nor in the air, but in the 
mind of the person whose ear is struck by the vibratory 
motion of the air, produced by a sonorous body. 

To convince you that sound does not exist in sonorous 
bodies, but that air or some other vehicle is necessary to 
its production, endeavour to ring the little bell, after I 
have suspended it under a receiver in the air-pump, from 
which I shall exhaust the air 

Caroline. This is indeed very strange : though I agi- 
tate it so violently, it does not produce the least sound. 

Mrs. B. By exhausting the receiver, I have cut off 
the communication between the air and the bell ; the lat- 
ter, therefore, cannot impart its motion to the air. 

Caroline. Are you sure that it is not the glass, which 
covers the bell, that prevents our hearing it? 

Mrs. B. That you may easily ascertain by letting the 
air into the receiver, and then ringing the bell. 

Caroline. Very true : I can hear it now almost as loud 
as if the glass did not cover it ; and I can no longer doubt 
but that air is necessary to the production of sound. 

Mrs. B. Not absolutely necessary, though by far the 
most common vehicle of sound. Liquids, as well as air, 
are capable of conveying the vibratory motion of a sono- 



7*24. What is sound, strictly speaking ? 725. How can it 

be shown that air is necessary in the production of sound ? 726. 

Why cannot a bell be heard in an exhausted receiver ? 727. 

lathe atmosphere the only conductor of sound ? 



178 ON WIND AND SOUND. 

rous body to the organ of hearing ; as sound can be heard 
under water. Solid bodies also convey sound, as I can 
soon convince you by a very simple experiment. I shall 
fasten this string by the middle round the poker ; now 
raise the poker from the ground by the two ends of the 
string, and hold one to each of your ears : — I shall now 
strike the poker with a key, and you will find that the 
sound is conveyed to the ear by means of the strings, in a 
much more perfect manner than if it had no other vehicle 
than the air. 

Caroline. That it is, certainly, for I am almost stun- 
ned by the noise. But what is a sonorous body, Mrs. B. 1 
for all bodies are capable of producing some kind of sound 
by the motion they communicate to the air. 

Mrs. B. Those bodies are called sonorous, which pro- 
duce clear, distinct, regular, and durable sounds, such as 
a bell, a drum, musical strings, wind instruments, &c. 
They owe this property to their elasticity ; for an elastick 
body, after having been struck, not only returns to its 
former situation, but having acquired momentum by its ve- 
locity, like the pendulum, it springs out on the opposite 
side. If I draw the string A B, which is made fast at 
both ends, to C, it will not only return to its original po- 
sition, but proceed onwards to D. 

This is its first vibration, at the end of which it will re- 
tain sufficient velocity to bring it to E, and back again to 
F, which constitutes its second vibration ; the third vibra- 
tion will carry it only to G and H, and so on till the re- 
sistance of the air destroys its motion. 

The vibration of a sonorous body gives a tremulous mo- 
tion to the air around it, very similar to the motion com- 
municated to smooth water when a stone is thrown into it. 
This first produces a small circular wave around the 
spot in which the stone falls ; the wave spreads, and 
gradually communicates its motion to the adjacent wa- 
ters, producing similar waves to a considerable extent. 
The same kind of waves is produced in the air by the 



728 What besides air convey the vibratory motion of sonorous 

bodies ? 729. What bodies are called sonorous ? 730. To 

what do they owe their sonorous property ? 731. How would 

you explain Fig. 6, plate XIV. as illustrating the production of 

sound ? 732. To what is the tremulous motion, given to the 

ar by a sonorous body, compared ? 



ON WIND AND SOUND. 179 

motion of a sonorous body, but with this difference, that 
as air is an elastick fluid, the motion does not consist of 
regularly extending waves, but of vibrations, and are com- 
posed of a motion forwards and backwards, similar to those 
of the sonorous body. They differ also in the one taking 
place in a plane, the other in all directions. The aerial 
undulations being spherical. 

Emily. But if the air moves backwards as well as for- 
wards, how can its motion extend so as to convey sound 
to a distance. 

i . Mrs. B. The first sphere of undulations which are 
produced immediately around the sonorous body, by 

| pressing against the contiguous air, condenses it. The 
condensed air, though impelled forward by the pressure, 
re-acts on the first set of undulations, driving them back 

- again. The second set of undulations which have been 
put in motion, in their turn communicate their motion, 

: and are themselves driven back byre-action. Thus there 

is a succession of waves in the air, corresponding with the 
• succession of waves in the water. 

Caroline. The vibrations of sound must extend much 

further than the circular waves in water, since sound is 

conveyed to a great distance. 

Mrs. B. The air is a fluid so much less dense than 

water, that motion is more easily communicated to it. 

The report of a cannon produces vibrations of the air 
. which extend to several miles around. 
., Emily. Distant sound takes some time to reach us, 

since it is produced at the moment the cannon is fired ; 

- and we see the light of the flash long before we hear the 
J report. 

Mrs. B. The air is immediately put in motion by the 
firing of a cannon ; but it requires time for the vibrations 
to extend to any distant spot. The velocity of sound is 
computed to be at the rate of 1142 feet in a second. 

Caroline. With what astonishing rapidity the vibra- 
tions must be communicated ! But the velocity of sound 
varies, I suppose, with that of the air which conveys it. 
If the wind sets towards us from the cannon, we must 
hear the report sooner than if it set the other way. 

733 If the air reverberate, how can its motion extend so as to 

convey sound to a distance ? 734. Why is motion more easily 

communicated to air than to water ? 735. Why do we see the 

flash of a cannon, at a distance, before we hear the report ? 

736. What is the computed velocity of sound ? 



180 ON WIND AND SOUND. 

Mrs. B. The direction of the wind makes less diffe- 
rence in the velocity of sound than you would imagine. 
If the wind sets from us, it bears most of the aerial waves 
away, and renders the sound fainter ; but it is not very 
considerably longer in reaching the ear than if the wind 
blew towards us. This uniform velocity of sound enables 
us to determine the distance of the object from which it 
proceeds ; as that of a vessel at sea firing a cannon, or 
that of a thunder cloud. If we do not hear the thunder 
till half a minute after we see the lightning, we conclude 
the cloud to be at the distance of six miles and a half. 

Emily* Pray how is the sound of an echo produced 2 

Mrs. B. When the aerial vibrations meet with an ob- 
stacle, having a hard and regular surface, such as a wall, 
or rock, they are reflected back to the ear and produce 
the same sound a second time ; but the sound will then 
appear to proceed from the object by which it is reflected. 
If the vibrations fall perpendicularly on the obstacle, they 
are reflected back in the same line ; if obliquely, the sound 
returns obliquely in the opposite direction, the angle of 
reflection being equal to the angle of incidence. 

Caroline. Oh, then, Emily, I now understand why the 
echo of my voice behind our house is heard so much 
plainer by you than it is by me, when we stand at oppo- 
site ends of the gravel walk. My voice, or rather, I should 
say, the vibrations of air it occasions, fall obliquely on the 
wall of the house, and are reflected by it to the opposite 
end of the gravel walk. 

Emily. Very true ; and we have observed that when 
we stand in the middle of the walk, opposite the house, 
the echo returns to the person who spoke. 

Mrs. B. Speaking-trumpets are constructed on the prin- 
ciple of the reflection of sound. The voice, instead of being 
diffused in the open air, is confined within the trumpet ; and 
the vibrations which spread and fall against the sides of the 
instrument, are reflected according to the angle of inci- 
dence, and fall into the direction of the vibrations which 
proceed straight forwards. The whole of the vibrations 
are thus collected into a focus ; and if the ear be situated 
in or near that spot, the sound is prodigiously increased. 

737. What effect has the direction of the wind on the velocitv 

of sound ? 738. To what practical purpose can we apply the 

uniform velocity of sound ? 739. How is the sound of an echo 

produced? 740. On what principle are speaking-trumpet 

constructed ? 



ON WIND AND SOUND. 181 

Figure 7, plate XIV. will give you a clearer idea of the 
speaking-trumpet : the reflected rays are distinguished 
from those of incidence, by being dotted ; and they are 
brought to a focus at F. The trumpet used by deaf per- 
sons acts on the same principle ; but as the voice enters 
the trumpet at the large instead of the small end of the 
instrument, it is not so much confined, nor the sound so 
much increased. 

Emily. Are the trumpets used as musical instruments 
also constructed on this principle 1 

Mrs. B. So far as their form tends to increase the 
sound, they are ; but, as a musical instrument, the trum- 
pet becomes itself the sonorous body, which is made to 
vibrate by blowing into it, and communicates its vibrations 
to the air. 

I will attempt to give you in a few words, some notion 
of the nature of musical sounds, which as you are fond of 
musick must be interesting to you. 

If a sonorous body be struck in such a manner, that its 
vibrations are all performed in regular times, the vibra- 
tions of the air will correspond with them ; and striking 
in the same regular manner on the drum of the ear, will 
produce the same uniform sensation on the auditory nerve 
and excite the same uniform idea in the mind ; or, in 
other words, we shall hear one musical tone. 

But if the vibrations of the sonorous body are irregular, 
there will necessarily follow a confusion of aerial vibra- 
tions ; for a second vibration may commence before the 
first is finished, meet it half way on its return, interrupt 
it in its course, and produce harsh jarring sounds which 
are called discords. 

Emily. But each set of these irregular vibrations, if 
repeated at equal intervals, would, I suppose, produce a 
musical tone. It is only their irregular succession which 
makes them interfere, and occasions discord. 



741. What does Figure 7, Plate XIV. represent ? 742. 

Where must the ear be situated in regard to the speaking-trumpet 
so as to receive an increased sound ? 743. How do the speak- 
ing-trumpets used by deaf persons differ from that in the figure ? 
744. How far is a trumpet used for a musical instrument con- 
structed on the above principle ? 745. How must a sonorous 

body be struck so that its vibrations produce in the mind the same 
uniform idea, or one musical tone ?— — 746 How are harsh jar- 
ring sounds or discords produced ? 
16 



182 ON WIND AND SOUND. 

Mrs. B. Certainly. The quicker a sonorous body vi- 
brates, the more acute, or sharp, is the sound produced. 

Caroline. But if I strike any one note of the piano- 
forte repeatedly, whether quickly or slowly, it always gives 
the same tone. 

Mrs. B. Because the vibrations of the same string, at 
the same degree of tension, are always of a similar dura- 
tion. The quickness or slowness of the vibrations relate 
to the single tones, not to the various sounds which they 
may compose by succeeding each other. Striking the 
note in quick succession, produces a more frequent repe- 
tition of the tone, but does not increase the velocity of the 
vibrations of the string. 

The duration of the vibrations of strings or chords de- 
pends upon their length, their thickness, or weight, and 
their degree of tension : thus, you find, the low bass notes 
are produced by long, thick, loose strings ; and the high 
treble notes by short, small, and tight strings. 

Caroline. Then the different length and size of the 
strings of musical instruments, serve to vary the duration 
of the vibrations, and consequently, the acuteness of gra- 
vity of the notes ? 

Mrs. B. Yes. Among the variety of tones, there are 
some which, sounded together, please the ear, producing 
what we call harmony, or concord. This arises from the 
agreement of the vibrations of the two sonorous bodies ; 
so that some of the vibrations of each strike upon the ear 
at the same time. Thus, if the vibrations of two strings 
are performed in equal times, the same tone is produced 
by both, and they are said to be in unison. 

Emily. Now, then, I understand why, when I tune 
my harp in unison with the piano-forte, I draw the strings 
tighter if it is too low, or loosen them if it is at too high a 
pitch ; it is in order to bring them to vibrate, in equal 
times, with the strings of the piano- forte. 

Mrs. B. But concord, you know, is not confined to 
unison ; for two different tones harmonize in a variety of 
cases. If the vibrations of one string (or sonorous body 
whatever) vibrate in double the time of another, the se- 
cond vibration of the latter will strike upon the ear at the 

747. On what does the acuteness or sharpness of a musical 

sound depend ? 748. On what does the duration of vibrations 

of strings or chords in musical instruments depend ? 749. 

Mow is harmony or concord in sounds produced ? 750. How is 

ooncord produced ? 



as opticus. 183 

same instant as the first vibration of the former ; and this 
is the concord of an octave. 

If the vibrations of two strings are as two to three, the 
second vibration of the first corresponds with the third vi- 
bration of the latter, producing the harmony called a fifth. 

Caroline. So, then, when I strike the key-note with 
its fifth, I hear every second vibration of one, and every 
third of the other at the same time 1 

Mrs. B. Yes; and the key-note struck with the 
fourth is likewise a concord, because the vibrations are as 
three to four. The vibrations of a major third with the 
key-note, are as four to five ; and those of a minor third, as 
five to six. 

There are other tones which, though they cannot be 
struck together without producing discord, if struck suc- 
cessively, give us the pleasure which is called melody. 
Upon these general principles the science of musick is 
founded ; but I am not sufficiently acquainted with it to 
enter any further into it.* 

We shall now, therefore, take leave of the subject of 
sound ; and, at our next interview, enter upon that of op- 
ticks, in which we shall consider the nature of vision, 
light, and colours. 

751. How is that species of harmony, callsd a fifth, produced ? 



CONVERSATION XIV 

ON OPTICKS. 



Of Luminous, Transparent, and Opaque Bodies; Of 
the Radiation of Light ; Of Shadows; Of the Reflec- 
tion of Light ; Opaque Bodies seen only by Reflected 
Light; Vision explained; Camera Obscura; Image 
of Objects on the Retina. 



CAROLINE. 



I long to begin our lesson to-day, Mrs. B., for I ex- 
pect that it will be very entertaining. 

* When musick is made by the use of strings, the air is struck 
by the body, and the sound is excited by the vibrations : when it is 
made by pipes, the body is struck by the air ; but as action and re- 
action are equal, the effect is the same in both cases. 



181 ON OPTICKS. 

Mrs. B. Opticks is certainly one of the most interest- 
ing branches of Natural Philosophy, but not one of the 
easiest to understand ; I must therefore beg that you will 
give me the whole of your attention. 

1 shall first inquire, whether you comprehend the mean- 
ing of a luminous body, an opaque body, and a transparent 
body. 

Caroline. A luminous body is one that shines ; an 
opaque .... 

Mrs. B. Do not proceed to the second, until we have 
agreed upon the definition of the first. All bodies that 
shine are not luminous ; for a luminous body is one that 
shines by its own light, as the sun, the fire, a candle, &c* 

Emily. Polished metal, then, when it shines with so 
much brilliancy, is not a luminous body ? 

Mrs. B. No, for it would be dark if it did not receive 
light from a luminous body ; it belongs, therefore, to the 
class of opaque or dark bodies, which comprehend all 
such as are neither luminous nor will admit the light to 
pass through them. 

Emily. And transparent bodies, are those which ad- 
mit the light to pass through them ; such as glass and 
water. 

Mrs. B. You are right. Transparent or pellucid 
bodies are frequently called mediums ; and the rays of 



* The direct light of the sun is calculated to be equal to that of 
6560 candles, placed at the distance of one foot from the object ; 
and that of the moon to the light of one candle at 7J feet distance ; 
of Jupiter at 1620 feet, and of Venus at 421 feet. Sir Isaac New- 
ton supposed rays of light to consist of exceedingly small particles, 
infinitely smaller than sand, moving from luminous bodies ; but 
later writers suppose them to consist of the undulations of an elas- 
tick medium, which fills all space, and which produces the sensa- 
tion of light to the eye, just as the vibrations of the air produce the 
sensation of sound to the ear. 



752. What is the science called that treats of vision ? 753. 

What is a luminous body ? 754. To what is the direct light of 

the sztn calculated to be equal ? 755. To what is the light of 

the moon — of Jupiter — and of Venus, respectively calculated to be 

equal f 756. What was Sir Isaac Keicton's opinion concerning 

the nature of light? 757. What is a modern opinion ? 

758 What are opaque bodies ? 759. What are transparent 

bodies ? 760. What are transparent bodies frequently called ? 



ON OPTICKS. 185 

light which pass through them, are said to be transmitted 
by them. / 

Light, when emanated from the sun, or any other lumi- 
nous body, is projected forwards in straight lines in every 
possible direction ; so that the luminous body is not only 
the general centre from whence all the rays proceed, but 
every point of it may be considered as a centre which ra- 
diates light in every direction. (Fig. 1. plate XV.) 

Emily, But do not the rays which are projected in 
different directions, and cross each other, interfere, and 
impede each other's course ? 

Mrs. B. Not at all. The particles of light are so ex- 
tremely minute, that they are never known to interfere 
with each other. A ray of light is a single line of light 
projected from a luminous body ; and a pencil of rays, is 
a collection of rays, proceeding from any one point of a 
luminous body, as fig. 2. 

Caroline. Is light then a substance composed of par- 
ticles like other bodies ? 

Mrs. B. This is a disputed point upon which I can- 
not pretend to decide. In some respects, light is obedi- 
ent to the laws which govern bodies ; in others it appears 
to be independent of them : thus, though its course is 
guided by the laws of motion, it does not seem to be in- 
fluenced by those of gravity.* It has never been disco- 
vered to have weight, though a variety of interesting ex- 
periments have been made with a view of ascertaining 
that point ; but we are so ignorant of the intimate nature 
of light, that an attempt to investigate it would lead us 
into a labyrinth of perplexity, if not of errour ; we shall 
therefore confine our attention to those properties of light 
which are well ascertained. 

Let us return to the examination of the effects of the 
radiation of light from a luminous body. Since the rays 
of light are projected in straight lines, when they meet 

761. In what manner is light produced from luminous bodies ? 

762. What is the reason that the progress of rays of light is 

not impeded by crossing each other ? 763. What is a ray of 

light ? 764. What is a pencil of rays ? 765. Is light a sub- 
stance composed of particles of matter like other bodies . p 766. 

In what respect is it subject to the laws of matter ? 767. In 

what respect is it not subject to the laws of matter ? 768. 

What is the consequence when rays of light fall upon an opaque 
body ? 

16 * 



186 ON OPT1CKS. 

With an opaque body through which they are unable to 
pass, they are stopped short in their course ; for they can 
not move in a curve line round the body. 

Caroline. No, certainly ; for it would require some 
other force besides that of projection, to produce motion 
in a curve line. 

Mrs. B. The interruption of the rays of light, by the 
opaque body, produces, therefore, darkness on the oppo- 
site side of it ; and if this darkness fall upon a wall, a 
sheet of paper, or any object whatever, it forms a shadow. 

Emily. A shadow then is nothing more than darkness 
produced by the intervention of an opaque body, which 
prevents the rays of light from reaching an object behind 
the opaque body. 

Caroline. Why then are shadows of different degrees 
of darkness : for I should have supposed, from your defi- 
nition of a shadow, that it would have been perfectly 
black ? 

Mrs. B. It frequently happens that a shadow is pro- 
duced by an opaque body interrupting the course of the 
rays from one luminous body, while light from another 
reaches the space where the shadow is formed, in which 
case the shadow is proportionally fainter. This happens 
if the opaque body be lighted by two candles : if you ex- 
tinguish one of them, the shadow will be both deeper and 
more distinct. 

Caroline. But yet it will not be perfectly dark. 

Mrs. B. Because it is still slightly illumined by light 
reflected from the walls of the room, and other surround- 
ing objects. 

You must observe, also, that when a shadow is pro- 
duced by the interruption of rays from a single luminous 
body, the darkness is proportional to the intensity of the 
light. 

Emily. I should have supposed the contrary ; for as 
the light reflected from surrounding objects on the sha- 
dow, must be in proportion to the intensity of the light, the 
stronger the light, the more the shadow will be illumined. 



769. What does this interruption produce in regard to the body ? 
770. What is a shadow ? 771 . Why are shadows of diffe- 
rent degrees of darkness ? 772. When a shadow is produced 

by the interruption of rays of light from a single opaque body ; to 
what is the darkness of the shadow proportional ? 



ON OPTICKS. 187 

Mrs. B. Your remark is perfectly just ; but as we have 
no means of estimating the degrees of light and of dark- 
ness but by comparison, the strongest light will appear to 
produce the deepest shadow. Hence a total eclipse of the 
sun occasions a more sensible darkness than midnight, 
as it is immediately contrasted with the strong light of 
noon-day. 

Caroline. The re-appearance of the sun after an 
eclipse, must, by the same contrast, be remarkably brilliant. 

Mrs. B. Certainly. There are several things to be 
observed in regard to the form and extent of shadows. 
If the luminous body A (fig. 3.) is larger than the opaque 
body B, the shadow will gradually diminish in size, till it 
terminate in a point. 

Caroline. This is the case with the shadows of the 
earth and the moon, as the sun which illumines them, is 
larger than either of those bodies. And why is it not the 
case with the shadows of terrestrial objects, which are 
equally illumined by the sun ? but their shadows, far from 
diminishing, are always larger than the object, and in- 
crease with the distance from it. 

Mrs. B. In estimating the effect of shadows, we must 
consider the apparent not the real dimensions of the lu- 
minous body ; and in this point of view, the sun is a small 
object compared with the generality of the terrestrial bo- 
dies which it illumines : and when the luminous body is 
less than the opaque body, the shadow will increase with 
the distance to infinity. All objects, therefore, which are 
apparently larger than the sun, cast a magnified shadow. 
This will be best exemplified, by observing the shadow of 
an object lighted by a candle. 

Emily. I have often noticed, that the shadow of my 
figure against the wall, grows larger as it is more distant 
from me, which is owing, no doubt, to the candle that 
shines on me being much smaller than myself? 

Mrs. B. Yes. The shado\v of a figure A, (fig. 4.) 
varies in size, according to the distance of the several sur- 
faces B C D E, on which it is described. 

773. Why does a total eclipse of the sun occasion a more sen- 
sible darkness than midnight ? 774. What will be the form of 

the shadow when a luminous body is larger than the opaque body 

upon which it shines ? 775. And why is it not the case with 

shadows of terrestrial objects, which are illumined by the sun ? 

776. When the luminous body is less than the opaque body, how 
does the shadow increase ? 777. Which figure illustrates this D 



188 ON OPTICKS. 

Caroline. I have observed, that two candles produce 
two shadows from the same object ; whilst it would ap- 
pear from what you said, that they should rather produce 
only half a shadow, that is to say, a very faint one. 

Mrs. B. The number of lights (indifferent directions) 
while it decreases the intensity of the shadow, increases 
their number, which always corresponds with that of the 
lights ; for each light makes the opaque body cast a diffe- 
rent shadow, as illustrated by fig. 5. It represents a ball 
A, lighted by three candles B, C, D, and you observe the 
light B produces the shadow 6, the light C the shadow c, 
and the light D the shadow d. 

Emily, I think we now understand the nature of 
shadows very well ; but pray what becomes of the rays of 
light which opaque bodies arrest in their course, and the 
interruption of which is the occasion of shadows ? 

Mrs. B. Your question leads to a very important pro- 
perty of light, Reflection. When rays of light encounter 
an opaque body, which they cannot traverse, part of them 
are absorbed by it, and part are reflected, and rebound 
just as an elastick ball which is struck against a wall. 

Emily. And is light in its reflection governed by the 
same laws as solid elastick bodies ? 

Mrs. B. Exactly. If a ray of light fall perpendicu- 
larly on an opaque body, it is reflected back in the same 
line, towards the point whence it proceeded. If it fall ob- 
liquely, it is reflected obliquely, but in the opposite direc- 
tion ; the angle of incidence being equal to the angle of 
reflection. You recollect that law in mechanicks ? 

Emily. Oh yes, perfectly. 

Mrs. B. If you will close the shutters, we shall ad- 
mit a ray of the sun's light through a very small aperture, 
and I can show you how it is reflected. I now hold this 
mirror, so that the ray shall fall perpendicularly upon it. 



778. How may more shadows than one be produced by a single 

opaque body ? 779. By which figure is this illustrated ? 

780. What is meant by the reflection of light? 781. Is all the 

light that falls upon an opaque body reflected ? 782. By what 

laws is the reflection of Kght governed ? 783. If a ray of light 

fall upon an opaque body perpendicularly, how will it be reflected ? 

784. How will it be reflected if it fall upon an opaque body 

obliquely ? 785. How does the angle of incidence compare 

with the angle of reflection P 



ON OPTICKS. 189 

Caroline. I see the ray which falls upon the mirror, 
but not that which is reflected by it. 

Mrs. B. Because its reflection is directly retrograde. 
The ray of incidence and that of reflection both being in 
the same line, though in opposite directions, are confound- 
ed together. 

Emily. The ray then which appears to us single, is 
really double, and is composed of the incident ray pro- 
ceeding to the mirror, and of the reflected ray returning 
from the mirror. 

Mrs. B. Exactly so. We shall now separate them 
by holding the mirror M, (fig. 6.) in such a manner, that 
the incident ray A B shall fall obliquely upon it — you see 
the reflected ray B C, is marching oft in another direc- 
tion. If we draw a line from the point of incidence B, 
perpendicular to the mirror, it will divide the angle of 
incidence from the angle of reflection, and you will see 
that they are equal. 

Emily. Exactly ; and now that you hold the mirror 
so that the ray falls more obliquely on it, it is also reflected 
more obliquely, preserving the equality of the angles of 
incidence and reflection. 

Mrs. B. It is by reflected rays only that we see 
opaque objects. Luminous bodies send rays of light im- 
mediately to our eyes, but the rays which they send to 
other bodies are invisible to us, and are seen only when 
they are reflected or transmitted by those bodies to our 
eyes. 

Emily. But have we not just seen the ray of light in 
its passage from the sun to the mirror, and its reflection ? 
yet in neither case were those rays in a direction to enter 
our eyes. 

Mrs. B. No. What you saw was the light reflected 
to your eyes by small particles of dust floating in the air, 
and on which the ray shone in its passage to and from the 
mirror. 

Caroline. Yet I see the sun shining on that house 
yonder, as clearly as possible. 

Mrs. B. Indeed you cannot see a single ray which 
passes from the sun to the house ; you see no rays but 

f&6. Which figure illustrates the manner in which light is re 

fleeted ? 787. By what rays do we see opaque bodies ? — 

788. How are we able to see light that falls upon an opaque body 
and is reflected, but not in a direction to meet the eye ? 



190 ON OPTICKS. 

those which enter your eyes ; therefore it is the rays 
which are reflected by the house to you, and not those 
which proceed from the sun to the house, that are visible 
to you. 

Caroline. Why then does one side of the house ap- 
pear to be in sunshine, and the other in the shade ? for if I 
cannot see the sun shine upon it, the whole of the house 
should appear in the shade. 

Mrs. B. That side of the house which the sun shines 
upon, reflects more vivid and luminous rays than the side 
which is in shadow, for the latter is illumined only by rays 
reflected upon it by other objects : these rays are therefore 
twice reflected before they reach your sight ; and as light 
is more or less absorbed by the bodies it strikes upon, 
every time a ray is reflected its intensity is diminished. 

Caroline. Still I cannot reconcile myself to the idea, 
that we do not see the sun's rays shining on objects, but 
only those which objects reflect to us. 

Mrs. B. I do not, however, despair of convincing you 
of it. Look at that large sheet of water ; can you tell why 
the sun appears to shine on one part of it only 1 

Caroline. No, indeed ; for the whole of it is equally 
exposed to the sun. This partial brilliancy of water has 
often excited my wonder ; but it has struck me more par- 
ticularly by moon-light. I have frequently observed a 
vivid streak of moon-shine on the sea, while the rest of 
the water remained in deep obscurity, and yet there was 
no apparent obstacle to prevent the moon from shining on 
every part of the water equally. 

Mrs. B. By moon-light the effect is more remarkable, 
on account of the deep obscurity of the other parts of the 
water ; while by the sun's light the effect is too strong for 
the eye to be able to contemplate it. 

Caroline. But if the sun really shines on every part of 
that sheet of water, why does not every part of it reflect 
rays to my eyes ? 

Mrs. B. The reflected rays are not attracted out of 
their natural course by your eyes. The direction of a 



789. Why does one side of an opaque body appear to be in the 
sun-shine and the other in the shade, when by not seeing the rays 
that fall upon the object, both sides of it would appear shaded ? 

790. What illustration is given to show that we only see the 

reflected light which falls upon different objects ? 



ON OPTICKS. 191 

reflected ray, you know, depends on that of the incident 
ray ; the sun's rays, therefore, which fall with various de- 
grees of obliquity upon the water, are reflected in direc- 
tions equally various ; some of these will meet your eyes, 
and you will see them, but those which fall elsewhere are 
invisible to you. 

Caroline. The streak of sunshine, then, which we 
now see upon the water, is composed of those rays which 
by their reflection happen to fall upon my eyes ? 

Mrs. B. Precisely. 

Emily. But is that side of the house yonder, which 
appears to be in shadow, really illumined by the sun, and 
its rays reflected another way. 

Mrs. B. No ; that is a different case from the sheet 
of water. That side of the house is really in shadow ; it 
is the west side, which the sun cannot shine upon till the 
afternoon. 

Emily. Those objects, then, which are illumined by 
reflected rays, and those which receive direct rays from the 
sun, but which do not reflect those rays towards us, ap- 
pear equally in shadow ? 

Mrs. B. Certainly ; for we see them both illumined 
by reflected rays. That part of the sheet of water, over 
which the trees cast a shadow, by what light do you see 
it? 

Emily. Since it is not by the sun's direct rays, it must 
be by those reflected on it from other objects, and which 
it again reflects to us. 

Caroline. But if we all see terrestrial objects by re- 
flected light, (as we do the moon,) why do they appear so 
bright and luminous ? I should have supposed that re- 
flected rays would have been dull and faint, like those of 
the moon. 

Mrs. B. The moon reflects the sun's light with as 
much vividness as any terrestrial object. If you look at 
it on a clear night, it will appear as bright as a sheet of 
water, the walls of a house, or any object seen by day-light 
and on which the sun shines. The rays of the moon 
are doubtless feeble, when compared with those of the 

791. Why is it that the whole surface of water on which the 

sun or moon shines does not appear illumined ? 792. How does 

the case of the sheet of water named, differ from that of the house 

on which the sun shines? 793. How are we enabled to see the 

moon 3 



192 0N0PT1CKS.' 

sun ; but that would not be a fair comparison, for the for 
mer are incident, the latter reflected rays. 

Caroline. True ; and when we see terrestrial object* 
by moonlight, the light has been twice reflected, and i: 
consequently proportionally fainter. 

Mrs. B. In traversing the atmosphere, the rays, both 
of the sun and moon, lose some of their light. For though 
the pure air is a transparent medium, which transmits the 
rays of light freely, we have observed, that near the sur- 
face of the earth it is loaded with vapours and exhalations, 
by which some portion of them are absorbed. 

Caroline. I have often noticed that an object on the 
summit of a hill appears more distinct than one at an 
equal distance in a valley, or on a plain ; which is owing, 
I suppose, to the air being more free from vapours in an 
elevated situation, and the reflected rays being conse- 
quently brighter. 

Mrs. B. That may have some sensible effect ; but 
when an object on the summit of a hill has a back ground 
of light sky, the contrast with the object makes its outline 
more distinct. 

Caroline. I now feel well satisfied that we see opaque 
objects only by reflected rays ; but I do not understand 
how these rays show us the objects from which they pro- 
ceed. 

Mrs. B. The rays of light enter at the pupil of the 
eye, and proceed to the retina, or optick nerve, which is 
situated at the back part of the eye-ball ; and there they 
describe the figure, colour, and (excepting size) form a per- 
fect representation of the object from which they proceed. 
We shall again close the shutters, and admit the light 
through the small aperture, and you will see a picture 
on the wall, opposite the aperture, similar to that which 
is delineated on the retina of the eye. 

Caroline. Oh, how wonderful ! there is an exact pic- 
ture in miniature of the garden, the gardener at work, the 

794. What effect is produced on the sun and moon's rays from 
traversing the atmosphere ? 795. What is there in the atmo- 
sphere that has a tendency to absorb the rays of light ? 796. 

Why is it that objects on a hill appear more distinct than at an 

equal distance from us in a valley ? 797. How is it that the 

rays of light give us an idea of the objects from which they pro- 

1 ? 798. What experiment illustrates the manner in which 

objects are delineated on the retina of the eye ? 



ON OPTICKS. 193 

trees blown about by the wind. The landscape would be 
perfect, if it were not reversed ; the ground being above 
and the sky beneath. 

Mrs. B. It is not enough to admire, you must under- 
stand this phenomenon, which is called a camera obscura, 
from the necessity of darkening the room, in order to ex- 
hibit it. 

This picture is produced by the rays of light reflected 
from the various objects in the garden, and which are ad- 
mitted through the hole in the window shutter. 

The rays from the glittering weathercock at the top of 
the alcove A, (pi. XVI. fig. 1.) represent it in this spot a; 
for the weathercock being much higher than the aperture in 
the shutter, only a few of the rays, which are reflected by 
it in an obliquely descending direction, can find entrance 
there. The rays of light, you know, always move in 
straight lines : those, therefore, which enter the room in 
a descending direction, will continue their course in the 
same direction, and will, consequently, fall upon the low- 
er part of the wall opposite the aperture, and represent 
the weathercock reversed in that spot, instead of erect in 
the uppermost part of the landscape. 

Emily. And the rays of light from the steps (B) of the 
alcove, in entering the aperture, ascend, and will describe 
those steps in the highest instead of the lowest part of the 
landscape. 

Mrs. B. Observe, too, that the rays coming from the 
alcove, which is to our left, describe it on the wall to 
the right ; while those which are reflected by the walnut 
tree C D, to our right, delineate its figure in the picture to 
the left c d. Thus the rays, coming in different directions, 
and proceeding always in right lines, cross each other at 
their entrance through the aperture : those which come 
above proceed below, those from the right go to the left, 
those from the left towards the right ; thus every object 
is represented in the picture, as occupying a situation the 
very reverse of that which it does in nature. 

Caroline. Excepting the flower-pot E F, which, though 

799. What is this illustration called ? 800. From what cir- 
cumstance does the camera obscura derive its name? 801. 

How would you explain Figure 1, plate XVI, as illustrating the 

camera obscura ? 802. Why do the objects exhibited by the 

camera obscura appear inverted ' 
17 



194 ON OPTICKS. 

its position is reversed, has not changed its situation in 
the landscape. 

Mrs. B. The flower-pot is directly in front of the 
aperture : so that its rays fall perpendicularly upon it, and 
consequently, proceed perpendicularly to the wall, where 
they delineate the object directly behind the aperture. 

Emily, And is it thus that the picture of objects is 
painted on the retina of the eye ?* 

Mrs. B. Precisely. The pupil of the eye, through 
which the rays of light enter, represents the aperture in the 
window-shutter ; and the image delineated on the retina, 
is exactly similar to the picture on the wall. 

Caroline. You do not mean to say, that we see only 
the representation of the object which is painted on the 
retina, and not the object itself? 

Mrs. B. If, by sight you understand that sense by 
which the presence of objects is perceived by the mind, 
through the means of the eyes, we certainly see only the 
image of those objects painted on the retina. 

Caroline. This appears to me quite incredible. 

Mrs. B. The nerves are the only part of our frame 
capable of sensation ; they appear, therefore, to be the 
instruments which the mind employs in its perceptions ; 
for a sensation always conveys an idea to the mind. Now 
it is known, that our nerves can be affected only by 
contact ; and for this reason the organs of sense cannot 
act at a distance ; for instance, we are capable of smell- 
ing only particles which are actually in contact with the 
nerves of the nose. We have already observed, that 
the odour of a flower consists in effluvia, composed of 
very minute particles, which penetrate the nostrils, and 



* Take off the sclerotica from the back part of the eye of an ox, 
or other animal, and place the eye in the hole of the window-shut- 
ter of a dark room, with its fore part towards the external objects *, 
a person in the room will, through the transparent coat, see the 
inverted image painted upon the retina. 



803- What part of the eye is represented by the aperture in the 

window-shutter ? 804. And to what is the picture on the wall 

in the camera obscura similar ? 805. Do we receive the sensa- 
tion of objects before u«, from the images formed on the retina of 

the eye, or direct from the objects themselves ? 806. How is 

an idea of visible objects conveyed to the mind ? 



ON OPTICKS. 195 

strike upon the olfactory nerves, which instantly convey 
the idea of smell to the mind. 

Emily. And sound, though it is said to be heard at a 
distance, is, in fact, heard only when the vibrations of the 
air, which convey it to our ears, strike upon the auditory 
nerve. 

Caroline. There is no explanation required to prove 
that the senses of feeling and of tasting are excited only 
by contact. 

Mrs, B. And I hope to convince you that the sense 
of sight is so likewise. The nerves, which constitute the 
sense of sight, are not different in their nature from those 
of the other organs ; they are merely instruments which 
convey ideas to the mind, and can be affected only on 
contact. Now since real objects cannot be brought to 
touch the optick nerve, the image of them is conveyed 
thither by the rays of light proceeding from real objects, 
which actually strike upon the optick nerve, and form that 
image which the mind perceives. 

Caroline. While I listen to your reasoning, I feel con- 
vinced ; but when I look upon the objects around, and 
think that I do not see them, but merely their image 
painted in my eyes, my belief is again staggered. I can- 
not reconcile myself to the idea, that I do not really see 
this book which I hold in my hand, nor the words which 
I read in it. 

Mrs. B. Did it ever occur to you as extraordinary, 
that you never beheld your own face. 

Caroline. No ; because I so frequently see an exact 
representation of it in the looking-glass. 

Mrs. B. You see a far more exact representation of 
objects on the retina of your eye : it is a much more per- 
fect mirror than any made by art. 

Emily. But is it possible, that the extensive landscape 
which I now behold from the window, should be repre- 
sented on so small a space as the retina of the eye ? 

Mrs. B. It would be impossible for art to paint so 
small and distinct a miniature ; but nature works with a 
surer hand and a more delicate pencil. That power, 
which forms the feathers of the butterfly, and the flowerets 
of the daisy, can alone portray so admirable and perfect 

807. How may the nerves which constitute the sense of sight 
be considered ? 



19(> ON OPTICKS. 

a miniature as that which is represented on the retina of 
the eye. 

Caroline, But, Mrs. B., if we see only the image of 
ohjects, why do we not see them reversed, as you showed 
us they were, in the camera obscura ? Is not that a strong 
argument against your theory ? 

Mrs. B. Not an unanswerable one, I hope. The 
image on the retina, it is true, is reversed, like that in the 
camera obscura ; as the rays, unless from a very small 
object, intersect each other on entering the pupil, in the 
same manner as they do on entering the camera obscura. 
The scene, however, does not excite the idea of being in- 
verted, because we always see an object in the direction 
of the rays which it sends to us. 

Emily. I confess I do not understand that. 

Mrs. B. It is, I think, a difficult point to explain 
clearly. A ray which comes from the upper part of an ob- 
ject describes the image on the lower part of the retina ; 
but experience having taught us that the direction of that 
ray is from above, we consider that part of the object it 
represents as uppermost. The rays proceeding from the 
lower part of an object fall upon the upper part of the re- 
tina ; but as we know their direction to be from below, 
we see that part of the object they describe as the lowest. 

Caroline. When I want to see an object above me, I 
look up ; when an object below me, I look down. Does 
not this prove that I see the objects themselves ? for if I 
beheld only the image, there would be no necessity for 
looking up or down, according as the object was higher 
or lower than myself. 

Mrs. B. I beg your pardon. When you look up to 
an elevated object, it is in order that the rays reflected 
from it should fall upon the retina of your eyes ; but the 
very circumstance of directing your eyes upwards con- 
vinces you that the object is elevated, and teaches you to 
consider as uppermost the image it forms on the retina, 
though it is, in fact, represented in the lowest part of it. 

When you look down upon an object, you draw your 
conclusion from a similar reasoning ; it is thus that we 
see all objects in the direction of the rays which reach 
our eyes. 

808. If objects are seen only by their pictures on the retina of 
the eye, why do they not appenr reversed, as in the camera obscU" 



ON THE ANGLE OP VISION. 197 

But I have a further proof in favour of what I have ad- 
vanced, which I hope will remove your remaining doubts ; 
I shall, however, defer it till 'our next meeting, as the les- 
son has been sufficiently long to-day. 



CONVERSATION XV. 

OPTICKS CONTINUED. 

ON THE ANGLE OF VISION, AND THE REFLECTION OF 
MIRRORS. 

Angle of Vision; Reflection of Plain Mirrors ; Reflection 
of Convex Mirrors ; Reflection of Concave Mirrors. 

CAROLINE. 

Well, Mrs. B., I am very impatient to hear what fur- 
ther proofs you have to offer in support of your theory. 
You must allow that it was rather provoking to dismiss us 
as you did at our last meeting. 

Mrs. B. You press so hard upon me with your objec- 
tions, that you must give me time to recruit my forces. 
Can you tell me, Caroline, why objects at a distance ap- 
pear smaller than they really are ? 

Caroline. I know no other reason than their distance. 

Mrs. B. I do not think I have more cause to be sa- 
tisfied with your reasons than you appear to be with mine. 
We must refer again to the camera obscura to account 
for this circumstance ; and you will find, that the different 
apparent dimensions of objects at different distances pro- 
ceed from our seeing, not the objects themselves, but 
merely their image on the retina. Fig. 1, plate XVII. 
represents a row of trees, as viewed in the camera obscura. 
I have expressed the direction of the rays, from the ob- 
jects to the image, by lines. Now, observe, the ray which 
comes from the top of the nearest tree, and that which 
comes from the foot of the same tree, meet at the aperture, 

809. Why do objects appear smaller at a distance than they 

really are ? 810. What is an angle of vision ? 811. Which 

figure illustrates the angle of vision ? 812. How would you 

explain that figure in reference to the effect that distance has on 
the apparent size of an object ? 

17* 



198 ON THE ANGLE OP VISION. 

forming an angle of about 25 degrees ; this is called the an- 
gle of vision, under which we see the tree. These rays 
cross each other at the aperture, forming equal angles on 
each side of it, and represent the tree inverted in the 
camera obscura. The degrees of the image are conside- 
rably smaller than those of the object, but the proportions 
are perfectly preserved. 

Now let us notice the upper and lower ray, from the 
most distant tree ; they form an angle of not more than 
twelve or fifteen degrees, and an image of proportional 
dimensions. Thus, two objects of the same size, as the 
two trees of the avenue, form figures of different sizes in 
the camera obscura, according to their distance ; or, in 
other words, according to the angle of vision under which 
they are seen. Do you understand this ? 

Caroline. Perfectly. 

Mrs. B. Then you have only to suppose that the re- 
presentation in the camera obscura is similar to that on 
the retina. 

Now since objects in the same magnitudes appear to 
be of different dimensions, when at different distances 
from us, let me ask you, which it is that we see ; the 
real objects, which we know do not vary in size, or the 
images, which we know do vary according to the angle of 
vision under which we see them 1 

Caroline. I must confess, that reason is in favour of 
the latter. But does that chair at the further end of the 
room form an image on my retina much smaller than this 
which is close to me ? they appear exactly of the same 
size. 

Mrs. B. I assure you they do not. The experience 
we acquire by the sense of touch corrects the errours of 
our sight with regard to objects within our reach. You 
are so perfectly convinced of the real size of objects 
which you can handle, that you do not attend to their 
apparent difference. 

Does that house appear to you much smaller than when 
you are close to it ? 

Caroline. No, because it is very near us. 

Mrs. B. And yet you can see the whole of it through 
one of the windows of this room. The image of the house, 
on your retina, must, therefore, be smaller than that of 

813. To what is the size of the anjrle of vision proportioned ? 



ON THE ANGLE OP VISION. 199 

the window through which you see it. It is your know- 
ledge of the real size of the house which prevents your 
attending to its apparent magnitude. If you were accus- 
tomed to draw from nature, you would be fully aware of this 
difference. 

Emily, And pray, what is the reason that, when we 
look up an avenue, the trees not only appear smaller as 
they are more distant, but seem gradually to approach 
each other till they meet in a point ? 

Mrs. B. Not only the trees, but the road which sepa- 
rates the two rows, forms a small visual angle, in propor- 
tion as it is more distant from us ; therefore the width of 
the road gradually diminishes as well as the size of the 
trees, till at length the road apparently terminates in a 
point, at which the trees seem to meet. 

But this effect of the angle of vision will be more fully 
illustrated by a little model of an avenue, which I have 
made for that purpose. It consists of six trees, leading to 
a hexagonal temple, and viewed by an eye, on the retina 
of which the picture of the objects is delineated. 

I beg that you will not criticise the proportions ; for 
though the eye is represented the size of life, while the 
trees are not more than three inches high, the dispropor- 
tion does not affect the principle, which the model is in- 
tended to elucidate. 

Emily. The threads which pass from the objects 
through the pupil of the eye to the retina, are, I suppose, 
to represent the rays of light which convey the image of 
the objects to the retina ? 

Mrs. B. Yes. I have been obliged to limit the rays 
to a very small number, in order to avoid confusion ; 
there are, you see, only two from each tree. 

Caroline. But as one is from the summit, and the 
other from the foot of the tree, they exemplify the diffe- 
rent angles under which we see objects at different dis- 
tances, better than if there were more. 

Mrs. B. There are seven rays proceeding from the 
temple, one from the summit, and two from each of the an- 
gles that are visible to the eye, as it is situated ; from 

814. Why are we not deceived as to the size of objects if the 
size of their images on the retina of the eye is varied by the dis- 
tance the objects are from us ? 815. Why does a road or any 

avenue appear to diminish in width, till at length it apparently 

terminates in a point ? 816. What is the reason that objects 

viewed in front appear larger than when viewed obliquely : 



200 ON THE ANGLE OP VISION. 

these you may form a just idea of the difference of the an- 
gle of vision of objects viewed obliquely, or in front ; for 
though the six sides of the temple are of equal dimen- 
sions, that which is opposite to the eye is seen under a 
much larger angle than those which are viewed obliquely. 
It is on this principle that the laws of perspective are 
founded. 

Emily. I am very glad to know that, for I have lately 
begun to learn perspective, which appeared to me a very 
dry study ; but now that I am acquainted with the princi- 
ples on which it is founded, I shall find it much more in 
teresting. 

Caroline. In drawing a view from nature, then, we 
do not copy the real objects, but the image they form on 
the retina of our eyes ? 

Mrs. B. Certainly. In sculpture, we copy nature as 
she really exists ; in painting, we represent her as she ap- 
pears to us. It was on this account that I found it diffi- 
cult to explain by a drawing the effects of the angle of 
vision, and was under the necessity of constructing a mo- 
del for that purpose. 

Emily. I hope you will allow us to keep this model 
some time, in order to study it more completely, for a 
great deal may be learned from it ; it illustrates the na- 
ture of the angle of vision, the apparent diminution of 
distant objects, and the inversion of the image on the re- 
tina. But pray, why are the threads that represent the 
rays of light, coloured, the same as the objects from which 
they proceed ] 

Mrs. B. That is a question which you must excuse 
my answering £t present, but I promise to explain it to 
you in due time. 

I consent very willingly to your keeping the model, on 
condition that you will make an imitation of it, on the 
same principle, but representing different objects. 

We must now conclude the observations that remain to 
be made on the angle of vision. 

If an object, with an ordinary degree of illumination, 
does not subtend an angle of more than two seconds of a 

817. On^what principle are the laws of perspective founded? 
818. In drawing a picture of any object what are we to fol- 
low ? fel9. How is nature to be exhibited in sculpture ? 

820. How is it to be represented in painting ? 821. When 

are objects invisible ? 



ON THE ANGLE OF VISION. 20x 

degree, it is invisible. There are consequently two cases 
in which objects may be invisible, either if they are too 
small, or so distant as to form an angle less than two se- 
conds of a degree. 

In like manner, if the velocity of a body does not ex- 
ceed 20 degrees in an hour, its motion is imperceptible. 

Caroline. A very rapid motion may then be imper- 
ceptible, provided the distance of the moving body is suffi- 
ciently great. 

Mrs. B. Undoubtedly ; for the greater its distance, 
the smaller will be the angle under which its motion will 
appear to the eye. It is for this reason that the motion 
of the celestial bodies is invisible, notwithstanding their 
immense velocity. 

Emily. I am surprised that so great a velocity as 20 
degrees an hour should be invisible. 

Mrs. B. The real velocity depends altogther on the 
space comprehended in each degree ; and this space de- 
pends on the distance of the object, and the obliquity of 
its path. Observe, likewise, that we cannot judge of the 
velocity of a body in motion unless we know its distance ; 
for supposing two men to set off at the same moment from 
A and B, (fig. 2.) to walk each to the end of their respec- 
tive lines C and D : if they perform their walk in the 
same space of time, they must have proceeded at a very 
different rate, and yet to an eye situated at E, they will 
appear to have moved with equal velocity : because they 
will both have gone through an equal number of degrees, 
though over a very unequal length of ground. Sight is an 
extremely useful sense no doubt, but it cannot always be 
relied on, it deceives us both in regard t& the size and 
the distance of objects ; indeed our senses would be very 
liable to lead us into errour, if experience did not set us 
right. 

Emily. Between the two, I think that we contrive to 
acquire a tolerably accurate idea of objects. 

Mrs. B. At least sufficiently so for the general pur- 
poses of life. To convince you how requisite experience 

822. What must be the velocity that its motion be perceptible ? 

» 823. Why is the motion of the celestial bodies imperceptible ? 

824. What is necessary for us to know in order to judge of 

the velocity of a moving body ? 825. In what respects may the 

sense of sight deceive us ? 826. By what are the errours into 

which we may be led by the senses to be corrected ? 



202 ON THE ANGLE OF VISION. 

is to correct the errours of sight, I shall relate to you the 
case of a young man who was blind from his infancy, and 
who recovered his sight at the age of fourteen, by the ope- 
ration of couching. At first he had no idea either of the 
size or distance of objects, but imagined that every thing 
he saw touched his eyes ; and it was not till after having 
repeatedly felt them, and walked from one object to ano- 
ther that he acquired an idea of their respective dimen- 
sions, their relative situations, and their distances. 

Caroline. The idea that objects touched his eyes, is 
however not so absurd as it at first appears ; for if we 
consider that we see only the image of objects, this image 
actually touches our eyes. 

Mrs. B. That is doubtless the reason of the opinion 
he formed, before the sense of touch had corrected his 
judgment. 

Caroline. But since an image must be formed on the 
retina of each of our eyes, why do we not see objects 
double ? 

31: s. B. The action of the rays on the optick nerve 
of each eye is so perfectly similar, that they produce but 
a single sensation ; the mind therefore receives the same 
idea, from the retina of both eyes, and conceives the ob- 
ject to be single. 

Caroline. This is difficult to comprehend, and, I 
should think, can be but conjectural. 

Mrs. B. I can easily convince you that you have a 
distinct image of an object formed on the retina of each 
eye. Look at the bell-rope, and tell me, do you see it to 
the right or the left of the pole of the fire-skreen ? 

Caroline. % little to the right of it. 

Mrs. B. Then shut your right eye, and you will see 
it to the left of the pole. 

Caroline. That is true indeed ! 

Mrs. B. There are evidently two representations of 
the bell-rope in different situations, which must be owing 
to an image of it being formed on both eyes ; if the action 
of the rays therefore on each retina were not so perfectly 
similar as to produce but one sensation, we should see 

827. How would objects appear as to distance, to one who had 

always been blind, on first being made to see ? .828. Why 

would they seem to touch the eye ? 829. If the image of an 

object is formed on the retina of each eye, why does not the object 
appear double ? 



REFLECTING MIRRORS. 203 

double, and we find that to be the case with many persons 
who are afflicted with a disease in one eye, which pre- 
vents the rays of light from affecting it in the same man- 
ner as the other. 

Emily. Pray, Mrs. B., when we see the image of an 
object in a looking-glass, why is it not inverted as in the 
camera obscura. and on the retina of the eye ? 

Mrs. B. Because the rays do not enter the mirror by 
a small aperture., and cross each other, as they do at the 
orifice of a camera obscura. or the pupil of the eye. 

When you view yourself in a mirror, the rays from 
your eyes fall perpendicularly upon it, and are reflected 
in the same line ; the image is therefore described behind 
the glass, and is situated in the same manner as the ob- 
ject before it. 

Emily. Yes, I see that it is ; but the looking-glass is 
not nearly so tall as I am : how is it therefore that I can 
see the whole of my figure in it ? 

Mrs. B. It is not necessary that the mirror should be 
more than half your height, in order that you may see the 
whole of your person in it (fig. 3.) The ray of light C D 
from your eye. which falls perpendicularly on the mirror 
B D, will be reflected back in the same line ; but the ray 
from your feet will fall obliquely on the mirror, for it 
must ascend in order to reach it ; it will therefore be re- 
flected in the line D A : and since we view objects in the 
direction of the reflected rays, which reach the eye. and 
that the image appears at the same distance behind the 
mirror that the object is before it. we must continue the 
line A D to E. and the line C D to F, at the termination 
of which, the image will be represented. 

Emily. Then I do not understand why I should not 
see the whole of my person in a much smaller mirror, for 
a ray of light from my feet would always reach it, though 
more obliquely. 

Mrs. B. True ; but the more obliquely the ray falls 
on the mirror, the more obliquely it will be reflected ; 



830. When we see the image of an object in a looking-glass. 

why doe^s it not appear inverted, as in the camera obscura ? 

831. Y\ hat must be the height of a looking-glass, in order for one 
whole person in it ? — — 832. How would you explain 

Fig. 3. of plate XVII. ? 833. Why may we not see ourselves 

entire, in a looking-glass less than half our*height : 



204 REINFECTING MIRRORS. 

the ray would therefore be reflected above your head, and 
you could not see it. This is shown by the dotted line, 
(fig. 3.) 

Now stand a little to the right of the mirror, so that 
the rays of light from your figure may fall obliquely on 
it — 

Emily. There is no image formed of me in the glass 
now. 

Mrs. B. I beg your pardon, there is : but you cannot 
see it, because the incident rays falling obliquely on the 
mirror will be reflected obliquely in the opposite direc- 
tion, the angles of incidence and of reflection being equal. 
Caroline, place yourself in the direction of the reflected 
rays, and tell me whether you do not see Emily's image 
in the glass ? 

Caroline. Let me consider. In order to look in the 
direction of the reflected rays, I must place myself as 
much to the left of the glass as Emily stands to the right 
of it. Now I see her image, but it is not straight before 
me, but before her ; and appears at the same distance 
behind the glass, as she is in front of it. 

Mrs, B. You must recollect, that we always see ob- 
jects in the direction of the last rays which reach our eyes. 
Figure 4 represents an eye looking at the image of a vase 
reflected by a mirror ; it must see it in the direction of 
the ray A B, as that is the ray which brings the image to 
the eye : prolong the ray to C, and in that spot will the 
image appear. 

Caroline. I do not understand why a looking-glass re- 
flects the rays of light : for glass is a transparent body which 
should transmit them. 

Mrs. B. It is not the glass that reflects the rays which 
form the image you behold, but the mercury behind it. 
The glass acts chiefly as a transparent case, through which 
the rays find an easy passage. 

834. How is this shown by the figure ? 835. Why cannot 

a person see his own image in a looking-glass, if he stand to the 

right or left of it ? 836. If you stand obliquely to the right of 

the glass, why must another person stand just as much to the left 

of it, in order to see your image ? 837. When you stand at 

the right of the glass, and I stand at the left of it, why does your 

image appear directly opposite to yourself ? 838. How would 

yon illustrate this by the Figure ? 839. If glass is a transpa 

rent body, why will looking-glasses reflect light ? 



REFLECTION OF MIRRORS. 205 

Caroline. Why then should not mirrors be made sim- 
ply of mercury 1 

Mrs. B. Because mercury is a fluid. By amalgamat- 
ing it with tin-foil, it becomes of the consistence of paste, 
attaches itself to the glass, and fowns in fact a mercurial 
mirror, which would be much more perfect without its 
glass cover ; for the purest glass is never perfectly transpa- 
rent ; some of the rays therefore are lost during their pas- 
sage through it, by being either absorbed, or irregularly 
reflected. 

This imperfection of glass mirrors has introduced the 
use of metallick mirrors, for optical purposes. 

Emily. But since all opaque bodies reflect the rays 
of light, I do not understand why they are not all mir- 
rors. 

Caroline. A curious idea indeed, sister ; it would be 
very gratifying to see one's self in every object at which 
one looked. 

Mrs. B. It is very true that all opaque objects reflect 
light ; but the surface of bodies in general is so rough 
and uneven, that their reflection is extremely irregular, 
which prevents the rays from forming an image on the 
retina. This you will be able to understand better, when 
I shall explain to you the nature of vision, and the struc- 
ture of the eye. 

You may easily conceive the variety of directions in 
which rays would be reflected by a nutmeg grater, on ac- 
count of the inequality of its surface, and the number of 
holes with which it is pierced. All solid bodies resemble 
the nutmeg-grater in these respects, more or less ; and it 
is only those which are susceptible of receiving a polish, 
that can be made to reflect the rays with regularity. As 
hard bodies are of the closest texture, the least porous, 
and capable of taking the highest polish, they make the 
best mirrors ; none therefore are so well calculated for 
this purpose as metals. 

Caroline. But the property of regular reflection is not 



840. If the mercury reflect the light, why should not mirrors 

be made of that material ? 841. What description of mirrors 

more perfect than glass have been introduced ? 842. If all 

opaque bodies reflect light, why cannot we see ourselves as well 
when looking at any other object, as when viewing a mirror ? 
—843. What substances make the most perfect mirrors ? 
18 



206 REFLECTION OF CpNVEX MIRRORS. 

confined to this class of bodies ; for I have often seen my 
self in a highly polished mahogany table. 

Mrs. B. Certainly ; but as that substance is less du 
rable, and its reflection less perfect, than that of metals, 
I believe it would seldom be chosen for the purpose of a 
mirror. 

There are three kinds of mirrors used in opticks ; the 
plain or flat, which are the common mirrors we have just 
mentioned ; convex mirrors ; and concave mirrors. 
The reflection of the two latter is very different from that 
of the former. The plain mirror, we have seen, does not 
alter the direction of the reflected rays, and forms an 
image behind the glass exactly similar to the object be- 
fore it. A convex mirror has the peculiar property of 
making the reflected rays diverge, by which means it di- 
minishes the image ; and a concave mirror makes the 
rays converge, and, under certain circumstances, magni- 
fies the image. 

Emily. We have a convex mirror in the drawing- 
room, which forms a beautiful miniature picture of the ob- 
jects in the room ; and I have often amused myself with 
looking at my magnified face in a concave mirror. But 
I hope you will explain to us why the one enlarges, while 
the other diminishes the objects it reflects. 

Mrs. B. Let us begin by examining the reflection of 
a convex mirror. This is formed of a portion of the ex- 
teriour surface of a sphere. When several parallel rays 
fall upon it, that ray only, which, if prolonged, would 
pass through the centre or axis of the mirror, is perpen- 
dicular to it. In order to avoid confusion, I have in fig. 
1, plate XVIII. drawn only three parallel lines, A B, 
C D, E F, to represent rays falling on the convex mirror 
M N ; the middle ray, you will observe, is perpendicular 
to the mirror, the others fall on it obliquely. 

Caroline. As the three rays are parallel, why are they 
not all perpendicular to the mirror ? 

Mrs. B. They would be so to a flat mirror ; but as 

844. How many kinds of mirrors are there used in opticks ? 

845. What are they ? 846. How does a plain mirror 

exhibit an object? 847. How does a convex mirror exhibit 

an object ? 848. How does a concave mirror exhibit an ob- 
ject ? 849. Of what is the convex mirror formed ? 850. 

What does Fig. 1, plate XVIII. represent ? 851. W T hen seve- 
ral ray.-' fall upon a convex mirror, which one will be perpendicu- 
lar to 'it ? 



REFLECTION 0«pCONVEX MIRRORS. 207 

this is spherical, no ray can fall perpendicularly upon it 
which is not directed towards the centre of the sphere. 

Emily. Just as a weight falls perpendicularly to the 
earth when gravity attracts it towards the centre. 

Mrs. B. In order, therefore, that rays may fall per- 
pendicularly to the mirror at B and F, the rays must be 
in the direction of the dotted lines, which, you may ob- 
serve, meet at the centre O of the sphere, of which the 
mirror forms a portion. 

Now can you tell me in what direction the three rays, 
A B, C D, E F, will be reflected ? 

Emily. Yes, I think so : the middle ray falling per- 
pendicularly on the mirror, will be reflected in the same 
line : the two others falling obliquely will be reflected 
obliquely to G H ; for the dotted lines you have drawn are 
perpendiculars, which divide their angles of incidence 
and reflection. 

Mrs. B. Extremely well, Emily ; and since we see 
objects in the direction of the reflected ray, we shall see 
the image at L, which is the point at which the reflected 
rays, if continued through the mirror, would unite and 
form an image. This point is equally distant from the 
surface and centre of the sphere, and is called the imagi- 
nary focus of the mirror. 

Caroline. Pray what is the meaning of a focus ? 

Mrs. B. A point at which converging rays unite. 
And it is in this case called an imaginary focus ; be- 
cause the rays do not really unite at that point, but only 
appear to do so : for the rays do not pass through the mir- 
ror, since they are reflected by it. 

Emily. I do not yet understand why an object ap- 
pears smaller when viewed in a convex mirror. 

Mrs. B. It is owing to the divergence of the reflected 
rays. You have seen that a convex mirror converts, by 
reflection, parallel rays into divergent rays ; rays that 
fall upon the mirror divergent, are rendered still more so 

852. In what direction must rays fall on the convex mirror M, 

N, at the points B, F, so as to be perpendicular to it ? 853. 

YVhy will the rays A, E, in Fig. 1, plate XVIII. be reflected to the 

points G, H ? 854. Why would the image formed from these 

rays be seen at the point L ? 855. What is the relative situa- 
tion of the point L, and what is it called ? 856. What is a fo- 
cus ?- 857. Why is the point L called an imaginary focus ? 

-858. Why does an object appear smaller when viewed in a 

convex mirror ? 



208 REFLECTION OF CONCAVE MIRRORS. 

by reflection, and convergent rays are reflected either 
parallel, or less convergent. If then an object be placed 
before any part of a convex mirror, as the vase A B, fig. 
2. for instance, the two rays from its extremities, falling 
convergent on the mirror, will be reflected less conver- 
gent, and will not come to a focus till they arrive at C ; 
then an eye placed in the direction of the reflected rays, 
will see the image formed in (or rather behind) the mirror 
at a b. 

Caroline. But the reflected rays do not appear to me 
to converge less than the incident rays. I should have sup- 
posed that, on the contrary, they converged more, since 
they meet in a point. 

Mrs. B. They would unite sooner than they actually 
do, if they were not less convergent than the incident rays : 
for observe, that if the Incident rays, instead of being re- 
flected by the mirror, continued their course in their 
original direction, they would come to a focus at D, which 
is considerably nearer to the mirror than at C ; the image 
is therefore seen under a smaller angle than the object ; 
and the more distant the latter is from the mirror, the less 
is the image reflected by it. 

You will now easily understand the nature of the re- 
flection of concave mirrors. These are formed of a por- 
tion of the internal surface of a hollow sphere, and their 
peculiar property is to converge the rays of light. 

Can you discover, Caroline, in what direction the three 
parallel rays, A B, C D, E F, which fall on the concave 
mirror M N, (fig. 3.) are reflected 1 

Caroline. I believe I can. The middle ray is sent 
back in the same line, as it is in the direction of the axis 
of the mirror ; and the two others will be reflected 
obliquely, as they fall obliquely on the mirror. I must 
now draw two dotted lines perpendicular to their points 
of incidence, which will divide their angles of incidence 
and reflection; and in order that those angles may be 
equal, the two oblique rays must be reflected to L, where 
they will unite with the middle ray. 



859. How would you explain by the Figure, the manner in 
which a convex mirror makes an object appear smaller than it is ? 

860. Of what is a concave mirror formed ? 861. How 

would you explain Fig. 3, plate XVIII. as illustrating the manner 
in which parallel rays will be reflected ? 



REFLECTION OF CONCAVE MIRRORS. 209 

Mrs. B. Very well explained. Thus you see that 
when any number of parallel rays fall on a concave mir- 
ror, they are all reflected to a focus ; for in proportion as 
the rays are more distant from the axis of the mirror, they 
fall more obliquely upon it, and are more obliquely reflect- 
ed ; in consequence of which they come to a focus in the 
direction of the axis of the mirror, at a point equally dis- 
tant from the centre and the surface of the sphere, and 
this point is not an imaginary focus, as happens with the 
convex mirror, but is the true focus at which the rays 
unite. 

Emily. Can a mirror form more than one focus by 
reflecting rays 1 

Mrs. B. Yes. If rays fall convergent on a concave 
mirror, (fig. 4.) they are sooner brought to a focus, L, than 
parallel rays ; their focus is therefore nearer to the mir- 
ror M N. Divergent rays are brought to a more distant 
focus than parallel rays, as in fig. 5. where the focus is at 
L ; but the true focus of mirrors, either convex or con- 
cave, is that of parallel rays, which is equally distant from 
the centre, and the surface of the sphere. 

I shall now show you the reflection of real rays of light, 
by a metallick concave mirror. This is one made of 
polished tin, which I expose to the sun, and as it shines 
bright, we shall be able to collect the rays into a very 
brilliant focus. I hold a piece of paper where I imagine 
the focus to be situated ; you may see by the vivid spot 
of light on the paper, how much the rays converge : but 
it is not yet exactly in the focus ; as I approach the paper 
to that point, observe how the brightness of the-spot of light 
increases, while its size diminishes. 

Caroline. That must be occasioned by the rays be- 
coming closer together. I think you hold the paper just 
in the focus now, the light is so small and dazzling— Oh, 
Mrs. B., the paper has taken fire ! 

862. Upon what does the obliquity depend with which parallel 

rays fall upon the surface of a concave mirror ? 863. What is 

the focus of a concave mirror ? 864. What is the relative po- 
sition of the focus to a concave mirror ? 865. Is the focus of 

a con rave mirror real, or only imaginary as in the convex mirror ? 

866. Will the focus be in the same place whether the rays 

fall parallel or converging upon the mirror t 867. Which is 

most distant from the mirror ? 868. Which figure illustrates 

this? 869. Which will form the more distant focus from the 

mirror, divergent or parallel ravs ? 870. Which figures illus- 
trate this ? 

18 ■ 



210 REFLECTION OF CONCAVE MIRRORS. 

Mrs. B. The rays of light cannot be concentrated, 
without, at the same time, accumulating a proportional 
quantity of heat : hence concave mirrors have obtained 
the name of burning-mirrors. 

Emily. I have often heard of the surprising effects of 
burning-mirrors, and I am quite delighted to understand 
their nature. 

Caroline. It cannot be the true focus of the mirror at 
which the rays of the sun unite, for as they proceed from 
a point, they must fall divergent upon the mirror. 

Mrs. B. Strictly speaking, they certainly do. But 
when rays come from such an immense distance as the 
sun, their divergence is so trifling, as to be impercepti- 
ble ; and they may be considered as parallel : their point of 
union is, therefore, the true focus of the mirror, and there 
the image of the object is represented. 

Now that I have removed the mirror out of the influence 
of the sun's rays, if I place a burning taper in the focus, 
how will its light be reflected 1 (fig. 6.) 

Caroline. That, I confess, I cannot say. 

Mrs. B. The ray which falls in the direction of the 
axis of the mirror, is reflected back in the same line ; but 
let us draw two other rays from the focus, falling on the 
mirror at B and F ; the dotted lines are perpendicular to 
those points, and the two rays will therefore be reflected 
to A and E. 

Caroline. Oh, now I understand it clearly. The rays 
which proceed from a light placed in the focus of a con- 
cave mirror fall divergent upon it, and are reflected pa- 
rallel. It is exactly the reverse of the former experiment, 
in which the sun's rays fell parallel on the mirror, and were 
reflected to a focus. 

Mrs. B. Yes : when the incident rays are parallel, 
the reflected rays converge to a focus ; when, on the con- 
trary, the incident rays proceed from the focus, they are 
reflected parallel. This is an important law of opticks, 
and since you are now acquainted with the principles on 
which it is founded, I hope that you will not forget it. 

871. What are concave mirrors sometimes called? 872. 

Why are they called burning-glasses ? 873. Do the rays 

which come from the sun, on being reflected by a concave mirror, 

meet in the true focus of the mirror ? 874. If a burning taper 

is placed in the focus of a concave mirror, how will its light be re- 
flected ? 875. What is illustrated by Fig. 6, plate XVIII. ?— — 

876. What is mentioned as an important law in opticks relating 
to the falling of light upon mirrors ? 



THE REFRACTION OF LIGHT. 21 1 

Caroline. I am sure that we shall not. But, Mrs. B., 
you said that the image was formed in the focus of a con- 
cave mirror ; yet I have frequently seen glass concave 
mirrors, where the object has been represented within the 
mirror, in the same manner as in a convex mirror. 

Mrs. B. That is the case only, when the object is 
placed between the mirror and its focus ; the image then 
appears magnified behind, or, as you call it, within the 
mirror. 

Caroline. I do not understand why the image should 
be larger than the object. 

Mrs. B. It proceeds from the convergent property of 
the concave mirror. If an object, A B, (fig. 7.) be placed 
between the mirror and its focus, the rays from its extre- 
mities fall divergent on the mirror, and on being reflected, 
become less divergent, as if they proceeded from C : to 
an eye placed in that situation the image will appear mag- 
nified behind the mirror at a b 9 since it is seen under a 
larger angle than the object. 

You now, I hope, understand the reflection of light by 
opaque bodies. At our next meeting, we shall enter upon 
another property of light no less interesting, which is call- 
ed refraction. 



CONVERSATION XVL 

ON REFRACTION AND COLOURS. 

Transmission of Light by Transparent Bodies ; Refrac- 
tion ; Refraction of the Atmosphere ; Refraction of a 
Lens ; Refraction of the Prism ; Of the Colours 
of Rays of Light ; Of the Colours of Bodies. 

MRS. b. % 

The refraction of light will furnish the subject of to- 
day's lesson. 

Caroline. This is a property of which I have not the 
faintest idea. 

877. Where must the object be placed in regard to a concave 

mirror, in order that the image appear behind the mirror ? 

878. Why does the image in a concave mirror appear larger than 
the object ? 879. How is this illustrated by the figure ? 



212 THE REFRACTION OF LIGHT. 

Mrs. B. It is the effect which transparent mediums 
produce on light in its passage through them. Opaque 
bodies, you know, reflect the rays, and transparent bodies 
transmit them ; but it is found, that if a ray, in passing 
from one medium into another of different density, fall 
obliquely, it is turned out of its course. 

Caroline. It must then be acted on by some new power, 
otherwise it would not deviate from its first direction. 

Mrs. B. The power which causes the deviation of 
the ray appears to be the attraction of the denser medium. 
Let us suppose the two mediums to be air and water ; if 
a ray of light passes from air into water, it is more strong- 
ly attracted by the latter on account of its superiour den- 
sity. 

Emily. In what direction does the water attract the 
ray ? 

Mrs. B. It must attract it perpendicularly towards it 
in the same manner as gravity acts on bodies. 

If then a ray A B, (fig. 1, plate XIX.) fall perpendicu- 
larly on water, the attraction of the water acts in the same 
direction as the course of the ray : it will not therefore 
cause a deviation, and the ray will proceed straight on to 
E. But if it fall obliquely, as the ray C B, the water will 
attract it out of its course. Let us suppose the ray to 
have approached the surface of a denser medium, and 
that it there begins to be affected by its attraction ; this 
attraction, if not counteracted by some other power, would 
draw it perpendicularly to the water, at B ; but it is also 
impelled by its projectile force, which the attraction of 
the denser medium cannot overcome ; the ray, therefore, 
acted on by both these powers, moves in a direction be- 
tween them, and instead of pursuing its original course to 
D, or being implicitly guided by the water to E, proceeds 
towards F, so that the ray appears bent or broken. 
dp Caroline. I understand that very well ; and is not this 
the reason that oars appear bent in water ? 



880. What is meant by the refraction of light ? 881. When 

does refraction in light take place ? 882. What power causes 

the refraction of light ? 883. How would you illustrate the re- 
fraction of light by an explanation of Fig. 1, plate XIX. ? 884. 

Why does the ray C B descend to F instead of D or E in that 
figure ? 



THE REFRACTION OF LIGHT. 213 

Mrs. B. It is owing to the refraction of the rays re- 
flected by the oar ; but this is in passing from a dense to 
a rare medium, for you know that the rays, by means of 
which you see the oar, pass from water into air. 

Emily. But I do not understand why a refraction 
takes place when a ray passes from a dense into a rare 
medium ; I should suppose that it would be rather less, 
than more attracted by the latter. 

Mrs. B. And it is precisely on that account that the 
ray is refracted. C B, fig. 2, represents a ray passing ob- 
liquely from glass into water : glass being the denser me- 
dium, the ray will be more strongly attracted by that which 
it leaves than by that which it enters. The attraction of 
the glass acts in the direction A B, while the impulse of 
projection would carry the ray to F ; it moves therefore 
between these directions towards D. 

Emily. So that a contrary refraction takes place when 
a ray passes from a dense into a rare medium. 

Caroline. But does not the attraction of the denser 
medium affect the ray before it touches it 1 

Mrs. B. The distance at which the attraction of the 
denser medium acts upon a ray is so small as to be insen- 
sible ; it appears therefore to be refracted only at the point 
at which it passes from one medium to the other. 

Now that you understand the principle of refraction, I 
will show you the refraction of a real ray of light. Do 
you see the flower painted at the bottom of the inside of 
this tea-cup 1 (Fig. 3.) 

Emily. Yes. But now you have moved it just out of 
sight ; the rim of the cup hides it. 

Mrs. B. Do not stir. I will fill the cup with water, 
and you will see the flower again. 

Emily. I do indeed ! Let me try to explain this : 
when you drew the cup from me so as to conceal the flow- 
er, the rays reflected by it no longer met my eyes, but 
were directed above them ; but now that you have rilled 
the cup with water, they are refracted by the attraction 
of the water, and bent downwards so as again to enter my 
eyes. 

Mrs. B. You have explained it perfectly : Fig. 3. 

885. Why does a straight stick appear crooked when one end 

of it is immersed obliquely in the water ? 886. How would 

you explain Fig. 2, plate XIX. ? 887. Does the attraction of 

the denser medium affect the ray before it touches it ? 



214 THE REFRACTION OF LIGHT- 

will help to imprint it on your memory. You must ob- 
serve that when the flower becomes visible by the refrac- 
tion of the ray, you do not see it in the situation which it 
really occupies, but an image of the flower higher in the 
cup ; for as objects always appear to be situated in the 
direction of the rays which enter the eye, the flower will 
be seen in the direction of the reflected ray at B. 

Emily. Then when we see the bottom of a clear 
stream of water, the rays which it reflects being refracted 
in their passage from the water into the air, will make the 
bottom appear higher than it really is. 

Mrs. B. And the water will consequently appear 
more shallow. Accidents have frequently been occasion- 
ed by this circumstance ; and boys who are in the habit 
of bathing should be cautioned not to trust to the appa- 
rent shallowness of water, as it will always prove deeper 
than it appears ; unless indeed, they view it from a boat 
on the water, which will enable them to look perpendicu- 
larly upon it ; when the rays from the bottom passing per- 
pendicularly, no refraction will take place. 

The refraction of light prevents our seeing the heaven- 
ly bodies in their real situation ; the light they send to us 
being refracted in passing into the atmosphere, we see the 
sun and stars in the direction of the refracted ray ; as de- 
scribed in fig. 4, plate XIX. ; the dotted line represents 
the extent of the atmosphere, above a portion of the earth 
ERE: a ray of light coming from the sun S falls ob- 
liquely on it, at A, and is refracted to B : then since we 
see the object in the direction of the refracted ray, a spec- 
tator at B will see an image of the sun at C, instead of 
the real object as S. 

Emily. But if the sun were immediately over our 
heads, its rays falling perpendicularly on the atmosphere 
would not be refracted, and we should then see the real 
sun in its true situation. 

Mrs. B. You must recollect that the sun is vertical 
only to the inhabitants of the torrid zone ; its rays, there- 

888. How would you describe the experiment represented in 
Fig. 3, plate XIX. ? 889. Why does water appear more shal- 
low than it really is ? 890. In what situation may the bottom 

of water be viewed so as to appear of its real depth ? 891. Do 

we see the heavenly bodies in their real situation ? 892. Why 

do we not ? 893. By which Figure is this illustrated, and how 

would you describe the illustration given ? 894. In what si- 
tuation may the sun be seen in its true place ? 



THE REFRACTION OF LIGHT. 215 

fore, are always refracted in these climates. There is 
also another obstacle to our seeing the heavenly bodies in 
their real situations ; light, though it moves with extreme 
velocity, is about eight minutes and a half in its passage 
from the sun to the earth ; therefore, when the rays reach 
us, the sun must have quitted the spot he occupied on 
their departure ; yet we see him in the direction of those 
rays, and consequently in a situation which he had aban- 
doned eight minutes and a half before. 

Emily. When you speak of the sun's motion, you 
mean, I suppose, his apparent motion, produced by the 
diurnal motion of the earth ? 

Mrs. B. No doubt ; the effect being the same, whether 
it is our earth, or the heavenly bodies which move : it is 
more easy to represent things as they appear to be, than 
as they really are. 

Caroline. During the morning, then, when the sun is 
rising towards the meridian, we must (from the length of 
time the light is in reaching us) see an image of the sun 
below that spot which it really occupies. 

Emily. But the refraction of the atmosphere counter- 
acting this effect, we may perhaps, between the two, see 
the sun in its real situation. 

Caroline. And in the afternoon, when the sun is sink- 
ing in the west, refraction and the length of time which 
the light is in reaching the earth, will conspire to render 
the image of the sun higher than it really is. 

Mrs. B. The refraction of the sun's rays by the at- 
mosphere prolongs our days, as it occasions our seeing an 
image of the sun both before he rises and after he sets ; 
for below the horizon, he still shines upon the atmosphere, 
and his rays are thence refracted to the earth. So like- 
wise we see an image of the sun before he rises, the rays 
that previously fall upon the atmosphere being reflected to 
the earth.* 



* It is entirely owing to the reflection of the atmosphere that 
the heavens appear bright in the day time. For without it, only 
that part would be luminous in which the sun is placed ; and if 

895. How long is light in coming from the sun to the earth ? 
896. How would you explain the effect this has on the ap- 
parent situation of that luminary ? 897. What effect does the 

refraction of light from the atmosphere have on the length of our 

days?- 898. What would be the appearance of the heavens 

were it not for the atmosphere f 



216 THE REFRACTION OF LIGHT. 

Caroline. On the other hand we must recollect that 
light is eight minutes and a half on its journey ; so that, by 
the time it reaches the earth, the sun may perhaps be risen 
above the horizon. 

Emily. Pray, do not glass windows refract the light ? 

Mrs. B. They do ; but this refraction is not percep- 
tible, because, in passing through a pane of glass, the rays 
suffer two refractions, which being in contrary directions, 
produce the same effect, as if no refraction had taken 
place. 

Emily. I do not understand that. 

Mrs. B. Fig. 5, plate XIX. will make it clear to 
you : A A represents a thick pane of glass seen edgeways. 
When the ray B approaches the glass at C, it is refracted 
by it ; and instead of continuing its course in the same di- 
rection, as the dotted line describes, it passes through the 
pane to D ; at that point returning into the air, it is again 
refracted by the glass, but in a contrary direction to the 
first refraction, and in consequence proceeds to E. Now 
you must observe that the ray B C and the ray D E being 
parallel, the light does not appear to have suffered any 
refraction. 

Emily. So that the effect which takes place on the 
ray entering the glass, is undone on its quitting it. Or, 
to express myself more scientifically, when a ray of light 
passes from one medium into another, and through that 
into the first again, the two refractions being equal and in 
opposite directions, no sensibfc effect is produced. 

Mrs. B. This is the case when the two surfaces of 
the refracting medium are parallel to each other ; if they 
are not, the two refractions may be made in the same di- 
rection, as I shall show you. 



we eould live withou* air, and should turn our backs to the sun, the 
whole heavens would appear as dark as in the night. In this case, 
also, we should have no twilight, but a sudden transition from the 
brightest sunshine to dark, immediately upon the setting of the 
sun. 



899. In what manner would the changes of day and night then 
lake place ? 900. Is light refracted in passing through com- 
mon window-glass ? 901. Why then is not the refraction per 

ccptiblc ? 902. Which figure illustrates this ? 



THE REFRACTION OF LIGHT* 217 

When parallel rays (fig. 6.) fall on a piece of glass hav- 
ing a double convex surface, and which is called a Lens, 
that only which falls in the direction of the axis of the 
lens is perpendicular to the surface ; the other rays fall- 
ing obliquely, are refracted towards the axis, and will 
meet at a point beyond the lens, called its focus. 

Of the three rays, ABC, which fall on the lens D E, 
the rays A and C are refracted in their passage through 
it, to a and c, and on quitting the lens they undergo a se- 
cond refraction in the same direction which unites them 
with the ray B at the focus F. 

Emily. And what is the distance of the focus from 
the surface of the lens ? 

Mrs. B. The focal distance depends both upon the 
form of the lens, and of the refractive power of the sub- 
stance of which it is made ; in a glass lens, both sides of 
which are equally convex, the focus is situated nearly at 
the centre of the sphere of which the surface of the lens 
forms a portion ; it is at the distance, therefore, of the ra- 
dius of the sphere. 

There are lenses of various forms, as you will find de- 
scribed in fig, 1, plate XX. The property of those which 
have a convex surface is to collect the rays of light to a 
focus ; and of those which have a concave surface, on the 
contrary, to disperse them. For the rays A C falling on 
the concave lens X Y, (fig. 7, plate XIX. ,) instead of con- 
verging towards the ray B, which falls on the axis of the 
lens, will each be attracted towards the thick edges of the 
lens, both on entering arid quitting it, and will, therefore, 
by the first refraction, be made to diverge to a, c, and by 
the second to d, e. 

Caroline. And lenses which have one side flat and the 
other convex or concave, as A and B, fig. 1 , plate XX 
are, I suppose, less powerful in their refractions. 

Mrs. B. Yes ; they are called plano-convex, and 
plano-concave lenses ; the focus of the former is at the 

903. What is a lens ? 904. In parallel rays that pass through 

a lens what ones will be refracted ? 905. In what place will 

the refracted rays meet ? 906. Which figure illustrates this ? 

907. What is the distance of the focus from the surface of 

the lens ? 908. What is the property of a convex lens ? 

309. What is the property of a concave lens ? 910. What 

does Figure 7, Plate 19 illustrate ? 911, What is a piano-con- 

vex lens ? 

19 



218 ON REFRACTION AND COLOURS. 

distance of the diameter of a sphere, of which the convex 
surface of the lens forms a portion ; as represented in 
fig. 2, plate XX. The three parallel rays, ABC, are 
brought to a focus by the plano-convex lens, X Y at F. 

I must now explain to you the refraction of a triangular 
piece of glass, called a prism. (Fig. 3.) 

Emily. The three sides of this glass are flat ; it can- 
not therefore bring the rays to a focus ; nor do I suppose 
that its refraction will be similar to that of a flat pane of 
glass, because it has not two sides parallel ; I cannot 
therefore conjecture what effect the refraction of a prism 
can produce. 

Mrs. B. The refractions of the light, on entering and 
on quitting the prism, are both in the same direction. 
(Fig. 3.) On entering the prism P, the ray A is refracted 
from B to C, and on quitting it from C to D. 

I will show you this in nature ; but for this purpose it 
will be adviseable to close the window-shutters, and ad- 
mit, through the small aperture, a ray of light, which I 
shall refract by means of this prism. 

Caroline. Oh, what beautiful colours are represented 
on the opposite wall ! There are all the colours of the rain- 
bow, and with a brightness I never saw equalled. (Fig. 
4, plate XX.) 

Emily. I have seen an effect, in some respect similar 
to this, produced by the rays of the sun shining upon glass 
lustres ; but how is it possible that a piece of white glass 
can produce such a variety of brilliant colours ? 

Mrs. B. The colours are not formed by the prism, but 
existed in the ray previous to its refraction. 

Caroline. Yet, before its refraction, it appeared per- 
fectly white. 

Mrs. B. The white rays of the sun are composed of 
coloured rays, which, when blended together, appear co- 
lourless or white. 

Sir Isaac Newton, to whom we are indebted for the 
most important discoveries respecting light and colours, 

912. What is a plano-concave lens? 913. Where will be 

the focus of a plano-convex lens ? 914. What is illustrated bv 

figure 2, plate XX. ? 915. What is a prism? 916. What 

does figure 3, plate XX. represent ? 917. What is the design 

of figure 4, plate XX. ? 918. Are the different colours exhibited 

in that figure formed by the prism ? 919. Of what are tha 

white rays of the sun composed ? 920. To whom are we indebt- 
ed for the most important discoveries respecting light and colours : 



ON REFRACTION AND COLOURS. 219 

was the first who divided a white ray of light, and found 
it to consist of an assemblage of coloured rays, which 
formed an image upon the wall, such as you now see ex 
hibited, (fig. 4.) in which are displayed the following se- 
ries of colours : red, orange, yellow, green, blue, indigo, 
and violet. 

Emily. But how does a prism separate these coloured 
rays ? 

Mrs. B. By refraction. It appears that the coloured 
rays have different degrees of refrangibility ; in passing 
through the prism, therefore, they take different direc- 
tions according to their susceptibility of refraction. The 
violet rays deviate most from their original course ; they 
appear at one of the ends of the spectrum A B : contigu- 
ous to the violet, are the blue rays, being those which 
have somewhat less refrangibility : then follow, in succes- 
sion, the green, yellow, orange, and, lastly, the red, which 
are the least refrangible of the coloured rays. 

Caroline. I cannot conceive how these colours, mixed 
together, can become white. 

Mrs. B. That I cannot pretend to explain ; but it is 
a fact that the union of these colours, in the proportions in 
which they appear in the spectrum, produce in us the 
idea of whiteness. If you paint a card in compartments 
with these seven colours, and whirl it rapidly on a pin, it 
will appear white. 

But a more decisive proof of the composition of a white 
ray is afforded by re-uniting these coloured rays, and form- 
ing with them a ray of white light.* 



* The same conclusion may be drawn from the experiment of 
mixing together paints of the colours exhibited in the prism, and 
in proper proportions, which will form white. It is true the white 
."will not be of the resplendent kind ; but this will be owing to the 
colours mixed being less bright than those produced from the 
prism. 

921. What is the order of the colours displayed in the prism ? 

922. How does the prism separate these rays ? 923. To 

what is the different directions, taken by the different rays in pass- 
ing through a prism, owing ? 924. Which rays deviate most 

and which least from their original course in passing through a 

prism? 925. What fact is mentioned respecting a painted 

card, as proving that these seven colours united make white ? 

926. What experiment relating to this subject is mentioned in ths 
note f 



220 ON REFRACTION AND COLOURS. 

Caroline. If you can take a ray of white light to pieces, 
and put it together again, I shall be quite satisfied. 

Mrs. B. This can be done by letting the coloured 
rays, which have been separated by a prism, fall upon a 
lens, which will converge them to a focus ; and if, when 
thus re-united, we find that they appear white as -ey did 
before refraction, I hope that you will be convinced that 
the white rays are a compound of the several coloured rays. 
The prism P, you see, (fig. 5.) separates a ray of white 
light into seven coloured rays, and the lens L L brings 
them to a focus at F, where they again appear white. 

Caroline. You succeed to perfection : this is indeed a 
most interesting and conclusive experiment. 

Emily. Yet, Mrs. B., I cannot help thinking, that 
there may perhaps be but three distinct colours in the 
spectrum, red, yellow, and blue ; and that the four others 
may consist of two of these colours blended together ; for 
in painting, we find that by mixing red and yellow, we pro- 
duce orange ; with different proportions of red and blue, 
we make violet or any shade of purple ; and yellow and 
blue form green. Now it is very natural to suppose, that 
the refraction of a prism may not be so perfect as to se- 
parate the coloured rays of light completely, and that those 
which are contiguous in order of refrangibility may en- 
croach on each other, and by mixing produce the inter- 
mediate colours, orange, green, violet, and indigo. 

Mrs. B. Your observation is, I believe, neither quite 
wrong, nor quite right. Dr. Wollaston, who has refract- 
ed light in a more accurate manner than had been pre- 
viously done, by receiving a very narrow line of light on a 
prism, found that it formed a spectrum, consisting of rays 
of four colours only ; but they were not exactly those you 
have named as primitive colours, for they consisted of red, 
green, blue, and violet. A very narrow line of yellow 
was visible, at the limit of the red and green, which Dr 
Wollaston attributed to the overlapping of the edges of 
the red and green light. 



927. How can these colours once separated be again united ? 

923. Which figure illustrates this ? 929. Who has been 

very successful and accurate in experiments upon the refrac- 
tion of light ? 930 What did he suppose to be the primitive 

colours ? 



ON REFRACTION AND COLOURS. 221 

Caroline. But red and green, mixed together, do not 
produce yellow. 

Mrs. B. Not" in painting ; but it may be so in the 
primitive rays of the spectrum. Dr. Wollaston observed 
that, by increasing the breadth of the aperture by which 
the line of light was admitted, the space occupied by each 
coloured ray in the spectrum was augmented in proportion 
as each portion encroached on the neighbouring colour 
and mixed with it ; so that the intervention of orange and 
yellow, between the red and green, is owing, he supposes, 
to the mixture of these two colours, and the blue is blend- 
ed on the one side with the green, and on the other 
with the violet, forming the spectrum as it was originally 
observed by Sir Isaac Newton, and which I have just 
shown you. 

The rainbow, which exhibits a series of colours so ana- 
logous to those of the spectrum, is formed by the refraction 
of the sun's rays in their passage through a shower of 
rain, every drop of which acts as a prism, in separating 
the coloured rays as they pass through it.* 

Emily. Pray, Mrs. B., cannot the sun's rays be col- 
lected to a focus by a lens in the same manner as they are 
by a concave mirror 1 

Mrs. B. No doubt the same effect is produced by the 
refraction of a lens as by the reflection of a concave mir- 
ror : in the first, the rays pass through the glass and con- 
verge to a focus behind it ; in the latter, they are reflect- 
ed from the mirror, and brought to a focus before it. A 
lens, when used for the purpose of collecting the sun's 
rays, is called a burning glass. The sun now shines very 
bright ; if we let the rays fall on this lens you will per- 
ceive the focus. 



* That this is the true account of the formation of the rainbow- 
appears from the following considerations — 1. That a bow is never 
seen but when rain is falling, and the sun shining at the same 
time, and that the sun and bow are always in opposite parts of 
the heavens ; and, secondly, that the same appearance can be 
artificially represented by means of water thrown into the air, when 
the spectator is placed in a proper position with his back towards 
the sun. 



931. How is the rain-bow formed ? 932. From what con- 

siderations does it appear that the rain-bow is formed by the re- 
fraction of the sun's rays in their passage through a shower of 

rain f 933 When is a lens called a burning glass ? 

19 * 



222 ON REFRACTION AND COLOURS. 

Emily. Oh yes ; the point of union of the rays is very 
luminous. I will hold a piece of paper in the focus, and 
see if it will take fire. The spot of light is extremely 
brilliant, but the paper does not burn. 

Mrs. B. Try a piece of brown paper ; — that you see 
takes fire almost immediately. 

Caroline. This is surprising ; for the light appeared 
to shine more intensely on the white than on the brown 
paper. 

Mrs. B. The lens collects an equal number of rays to 
a focus, whether you hold the white or the brown paper 
there ; but the white paper appears more luminous in the 
focus, because most of the rays, instead of entering into 
the paper, are reflected by it ; and this is the reason that 
the paper is not burnt ; whilst on the contrary, the brown 
paper, which absorbs more light than it reflects, soon be- 
comes heated and takes fire. 

Caroline. This is extremely curious ; but vvhy should 
brown paper absorb more rays than white paper 1 

Mrs. B. I am far from being able to give a satisfac- 
tory answer to that question. We can form but mere 
conjecture on this point ; and suppose that the tendency 
to absorb, or reflect rays, depends on the arrangement of 
the minute particles of the body, and that this diversity of 
arrangement renders some bodies susceptible of reflecting 
one coloured ray, and absorbing the others ; whilst other 
bodies have a tendency to reflect all the colours, and others 
again, to absorb them all. 

Emily. And how do you know which colours bodies 
have a tendency to reflect, or which to absorb ? 

Mrs. B. Because a body always appears to be of the 
colour which it reflects ; for as we see only by reflected 
rays, it can appear but of the colour of those rays. 

Caroline. But we see aH bodies of their own natural 
colour, Mrs. B. ; the grass and trees, green ; the sky, 
blue ; the flowers, of various hues. 

Mrs. B. True : but why is the grass green ? because 
it absorbs all except the green rays ; it is therefore these 
only which the grass and trees reflect to our eyes, and 

934. Why will a piece of brown paper placed beneath a lens, 
which collects the sun's rays, take fire sooner than a piece of white 

paper r 935. What conjecture is given for the brown paper's 

absorbing more rays than the white ? 936. How do we know 

which colours bodies have a tendency to reflect, and which to ab- 
sorb ? 



ON REFRACTION AND COLOURS 223 

which makes them appear green. The sky and flowers, 
m the same manner, reflect the various colours of which 
they appear to us : the rose, the red rays ; the violet, the 
blue : the jonquil,, the yellow. &c 

Caroline. But these are the permanent colours of the 
grass and flowers, whether the sun's rays shine on them 
or not. 

Mrs. B. Whenever you see those colours, the flowers 
must be illumined by some light ; and light, from what- 
ever source it proceeds, is of the same nature, composed 
of the various coloured rays, which paint the grass, the 
flowers, and every coloured object in nature. 

Caroline. But, Mrs. B., the grass is green, and the 
flowers are coloured, whether in the dark or exposed to 
the light ? 

Mrs. B. Why should you think so ! 

Caroline. It cannot be otherwise. 

Mrs. B. A most philosophical reason indeed ! But, 
as I never saw them in the dark, you will allow me to dis- 
sent from your opinion. 

Caroline. What colour do you suppose them to be, 
then, in the dark ? 

Mrs. B. None at all : or black, which is the same 
thing. You can never see objects without light. Light 
is composed of colours, therefore there can be no light 
without colours ; and though every object is black, or 
without colour in the dark, it becomes coloured, as soon 
as it becomes visible. It is visible, indeed, but by the 
coloured rays which it reflects ; therefore we can see it 
only when coloured. 

Caroline. All you say seems very true,, and I know not 
what to object to it : yet it appears at the same time in- 
credible ! What,. Mrs. B.. are we all as black as negroes, 
in the dark ? you make me shudder at the thought. 

Mrs. B. Your vanity need not be alarmed at the idea, 
as you are certain of never being seen in that state. 

Caroline. That is some consolation, undoubtedly ; 
but what a melancholy reflection it is. that all nature 
which appears so beautifully diversified with colours 
should be one uniform mass of blackness ! 

Mrs. B. Is nature less pleasing for being coloured, as 
well as illumined by the rays of light : and are colours less 

937. Are cole utial properties of bodies ? 938. On 

what do thev depend ; 939 What colour do obiect< have in 

the dark - ' ' 



224 ON REFRACTION AND COLOURS. 

beautiful for being accidental, rather than essential pro- 
perties of bodies ? 

Providence appears to have decorated nature with the 
enchanting diversity of colours which we so much admire, 
for the sole purpose of beautifying the scene, and render- 
ing it a source of pleasurable enjoyment : it is an orna- 
ment which embellishes nature whenever we behold her. 
What reason is there to regret that she does not wear it 
when she is invisible ? 

Emily. I confess, Mrs. B., that I have had my doubts 
as well as Caroline, though she has spared me the pains 
of expressing them ; but I have just thought of an experi- 
ment, which, if it succeeds, will, I am sure, satisfy us 
both. It is certain, that we cannot see bodies in the dark, 
to know whether they have any colour. But we may place 
a coloured body in a ray of light, which has been refracted 
by a prism ; and if your theory is true, the body, of what- 
ever colour it naturally is, must appear of the colour of the 
ray in which it is placed ; for since it receives no other 
coloured rays, it can reflect no others. 

Caroline. Oh ! that is an excellent thought, Emily ; 
will you stand the test, Mrs. B. ? 

Mrs. B. I consent : but we must darken the room, 
and admit only the ray which is to be refracted ; other- 
wise, the white rays will be reflected on the body under 
trial from various parts of the room. With what do you 
choose to make the experiment ? 

Caroline. This rose : look at it, Mrs. B., and tell me 
whether it is possible to deprive it of its beautiful colour ? 

Mrs. B. We shall see. — I expose it first to the red 
rays, and the flower appears of a more brilliant hue ; but 
observe the green leaves — 

Caroline. They appear neither red nor green ; but of 
a dingy brown with a reddish glow ! 

Mrs. B. They cannot be green, because they have no 
green rays to reflect ; neither are they red, because 
green bodies absorb most of the red rays. But though 
bodies, from the arrangement of their particles, have a 
tendency to absorb some rays, and reflect others, yet it 



940. What experiment is proposed to prove that bodies appear 

of the colour of the particular ray in which they are placed ? 

941. Why is it necessary to darken the room in which the experi- 
ment is to be made ? 942. How would a green object appear 

placed in a red ray ? 



ON REFRACTION AND COLOURS. 225 

is not natural to suppose, that bodies are so perfectly uni- 
form in their arrangement, as to reflect only pure rays of 
one colour, and perfectly absorb the others ; it is found, 
on the contrary, that a body reflects, in great abundance, 
the rays which determine its colour, and the others in a 
greater or less degree, in proportion as they are nearer or 
further from its own colour, in the order of refrangibility. 
The green leaves of the rose, therefore, will reflect a few 
of the red rays which, blended with their natural black- 
ness, give them that brown tinge ; if they reflected none 
of the red rays, they would appear perfectly black. Now 
I shall hold the rose in the blue rays — 

Caroline. Oh, Emily, Mrs. B. is right ! look at the 
rose : it is no longer red, but of a dingy blue colour. 

Emily. This is the most wonderful of any thing we 
have yet learned. But, Mrs. B., what is the reason that the 
green leaves are of a brighter blue than the rose 1 

Mrs. B. The green leaves reflect both blue and yel- 
low rays, which produces a green colour. They are now 
in a coloured ray, which they have a tendency to reflect ; 
they, therefore, reflect more of the blue rays than the rose, 
(which naturally absorbs that colour,) and will, of course, 
appear of a brighter blue. 

Emily. Yet, in passing the rose through the different 
colours of the spectrum, the flower takes them more rea- 
dily than the leaves. 

Mrs. B. Because the flower is of a paler hue. Bodies 
which reflect all the rays are white ; those which absorb 
them all are black : between these extremes, the body 
appears lighter or darker, in proportion to the quantity of 
rays they reflect or absorb. This rose is of a pale red : 
it approaches nearer to white than black ; it therefore re- 
flects rays more abundantly than it absorbs them. 

Emily. But if a rose bus so strong a tendency to re- 
flect rays, I should have imagined that it would be of a 
deep red colour. 

943. Why would it appear of a brownish tinge ? 944. If 

a red object be placed in a blue ray how will it appear ? 945. 

Why does an object that is green placed in a blue ray appear of 
a brighter blue, than an object that is red when placed in the same 

coloured ray ? 946. In passing a red and green object through 

the different colours of the spectrum, why does the red one take 
them more readily than the green one ? 947. What bodies re- 
flect all the rays that fall on them ? 948, What ones absorb 

them ? 



226 ON REFRACTION AND COLOURS. 

Mrs. B. I mean to say, that it has a general tenden- 
cy to reflect rays. Pale coloured bodies reflect all the co- 
loured rays to a certain degree, which produces their pale- 
ness, approaching to whiteness ; but one colour they re- 
flect more than the rest ; this predominates over the white, 
and determines the colour of the body. Since, then, bo- 
dies of a pale colour in some degree reflect all the rays of 
light, in passing through the various colours of the spec- 
trum, they will reflect them all with tolerable brilliancy ; 
but will appear most vivid in the ray of their natural co- 
lour. The green leaves, on the contrary, are of a dark 
colour, bearing a stronger resemblance to black, than to 
white ; they have, therefore, a greater tendency to ab- 
sorb, than to reflect rays ; and reflecting very few of any 
but the blue and yellow rays, they will appear dingy in 
passing through the other colours of the spectrum. 

Caroline. They must, however, reflect great quan- 
tities of the green rays to produce so deep a colour. 

Mrs. B. Deepness or darkness of colour proceeds 
rather from a deficiency than an abundance of reflected 
rays. Remember that bodies are, of themselves, black ; 
and if a body reflects only a few green rays, it will appear 
of a dark green ; it is the brightness and intensity of the 
colour which show that a great quantity of rays are re- 
flected. 

Emily. A white body, then, which reflects all the 
rays, will appear equally bright in all the colours of the 
spectrum. 

Mrs. B. Certainly ; and this is easily proved by pass- 
ing a sheet of white paper through the rays of the spec- 
trum. 

Caroline. What is the reason that blue often appears 
green by candle-light ? 

Mrs. B. The light of a candle is not so pure as that 
of the sun ; it has a yellowish tinge, and when refracted 
by the prism, the yellow rays predominate ; and as blue 
bodies reflect the yellow rays in the next proportion, 
(being next in order of refrangibility,) the superabundance 
of yellow rays gives to blue bodies a greenish hue. 

Caroline. Candle-light must then give to all bodies a 
yellowish tinge, from the excess of yellow rays ; and yet 

949 To what is darkness of colour owing ? 950. What is 

the reason the blue often appears green by candle-light ? 951. 

Why do persons of a sallow complexion appear fairer or whiter 
by night, if the candle-light gives all objects a yellowish tinge? 



ON REFRACTION AND COLOUR. 227 

it is a common remark, that people of a sallow complexion 
appear fairer or whiter by candle-light. 

Mrs. B. The yellow cast of their complexion is not 
so striking, when every object has a yellow tinge. 
Emily. Pray, why does the sun appear red through a fog ? 

Mrs. B. It is supposed to be owing to the red rays hav- 
ing a greater momentum, which gives them power to tra- 
verse so dense an atmosphere. For the same reason, the 
sun generally appears red at rising and sitting : as the 
increased quantity of atmosphere, which the oblique rays 
must traverse, loaded with the mists and vapours which 
are usually formed at those times prevents the other rays 
from reaching us. 

Caroline. And, pray, why are the skies of a blue co- 
lour. 

Mrs. B. You should rather say, the atmosphere ; for 
the sky is a very vague term, the meaning of which it 
would be difficult to define philosophically. 

Caroline. But the colour of the atmosphere should be 
white, since all the rays traverse it in their passage to the 
earth. 

Mrs. B. Do not forget that we see none of the rays 
which pass from the sun to the earth, excepting those 
which meet our eyes ; and this happens only if we look 
8t the sun, and thus intercept the rays, in which case, you 
know, the sun appears white. The atmosphere is a trans- 
parent medium, through which the sun's rays pass freely 
to the earth ; but when reflected back into the atmosphere, 
their momentum is considerably diminished ; and they 
have not all of them power to traverse it a second time. 
The momentum of the blue rays is least ; these, there- 
fore, are the most impeded in their return, and are chiefly 
reflected by the atmosphere : this reflection is performed 
in every possible direction ; so that whenever we look at 
the atmosphere, some of these rays fall upon our eyes ; 
hence we see the air of a blue colour. If the atmosphere 
did not reflect any rays, though the objects on the sur- 
face of the earth would be illumined, the skies would ap- 
pear perfectly black. 

Caroline. Oh, how melancholy that would be ; and 
how pernicious to the sight, to be constantly viewing 

952. Why does the sun appear red in the morning and when 
seen through fog or clouds ? — — 953. Why does the sky or at- 
mosphere appear blue ? 954. How would the sky appear if the 

atmosphere reflected none of the rays of light ? 



228 ON REFRACTION AND COLOURS. 

bright objects against a black sky ! But what is the reason 
that bodies often change their colour ; as leaves which 
wither in autumn, or a spot of ink which produces an iron- 
mould on linen ? 

Mrs. B. It arises from some chemical change, which 
takes place in the internal arrangement of the parts, by 
which they lose their tendency to reflect certain colours, 
and acquire the power of reflecting others. A withered 
leaf thus no longer reflects the blue rays ; it appears, 
therefore, yellow, or has a slight tendency to reflect seve- 
ral rays which produce a dingy brown colour. 

An ink-spot on linen at first absorbs all the rays ; but 
exposed to the air, it undergoes a chemical change, and 
the spot partially regains its tendency to reflect colours, 
but with a preference to reflect the yellow rays, and such 
is the colour of the iron-mould. 

Emily. Bodies, then, far from being of the colour 
which they appear to possess, are of that colour which 
they have the greatest aversion to, which they will not in- 
corporate with, but reject and drive from them. 

Mrs. B. It certainly is so ; though I scarcely dare 
venture to advance such an opinion whilst Caroline is con- 
templating her beautiful rose. 

Caroline. My poor rose ! you are are not satisfied with 
depriving it of colour, but even make it have an aversion 
to it ; and I am unable to contradict you. 

Emily. Since dark bodies absorb more solar rays than 
light ones, the former should sooner be heated if exposed 
to the sun. 

Mrs. B. And they are found by experience to be so. 
Have you never observed a black dress to be warmer than a 
white one ? 

Emily. Yes, and a white one more dazzling : the 
black is heated by absorbing the rays, the white dazzling 
by reflecting them. 

Caroline. And this was the reason that the brown paper 
was burnt in the focus of the lens, whilst the white paper 
exhibited the most luminous spot, but did not take fire. 

Mrs. B. It was so. It is now full time to conclude 
our lesson. At our next meeting, I shall give you a de- 
scription of the eye. 

955. What is the reason that they often change their colour ? 

950. What dress is warmest, a Hack or a white one ? 957. 

Why is a black one warmest ' 958 Why is a white more daz- 
zling than a black dri 



opticks. 229 

CONVERSATION XVII. 

OPTICKS. 

ON THE STRUCTURE OF THE EYE, AND OPTICAL INSTRU- 
MENTS. 

Description of the Eye ; Of the Image on the Retina ; 
Refraction of the Humours of the Eye ; Of the Use of 
Spectacles ; Of the Single Microscope ; Of the Double 
Microscope ; Of the Solar Microscope ; Magick Lan- 
tern ; Refracting Telescope ; Refecting Telescope* 

MRS. B. 

The body of the eye is of a spherical form : (fig. 1, 
plate XXI.) It has two membraneous coverings ; the exter- 
nal one, a a a, is called the sclerotica ; this has a projec- 
tion in that part of the eye which is exposed to view, b b, 
which is called the cornea, because, when dried, it has 
nearly the consistence of very fine horn, and is sufficient- 
ly transparent for the light to obtain free passage through it. 

The second membrane which lines the cornea, and en- 
velopes the eye, is called the choroid, c c c; this has an 
opening in front, just beneath the cornea, which forms the 
pupil, d d, through which the rays of light pass into the 
eye. The pupil is surrounded by a coloured border, call- 
ed the iris, e e, which, by its muscular motion, always pre- 
serves the pupil of a circular form, whether it is expanded 
in the dark, or contracted by a strong light. This you 
will understand better by examining fig. 2. 

Emily. I did not know that the pupil was susceptible 
of varying its dimensions. 

Mrs. B. The construction of the eye is so admirable, 
that it is capable of adapting itself, more or less, to the 
circumstances in which it is placed. In a faint light the 

959. What is the form of the body of the eye ? 960. Which 

figure represents an eye ? 961. What is the external covering 

of the eye called ? 962. Which part of the eye is called the 

cornea ? 963. From what does the cornea take its name ? 

964. What part of the eye is called the choroid ? 965. What 

part of the figure represents the choroid ? 966. What is that 

part of the eye called through which the light passes ? 967. 

By what part of the figure is the pupil represented ? 968. 

By what is the pupil of the eye surrounded ? 969. What repre- 
sents the iris in the figure ? 970. Is the pupil of the eye al- 
ways of the same size ? 

20 



230 OPTICKS* 

pupil dilates so as to receive an additional quantity of rays, 
and in a strong light it contracts, in order to prevent the 
intensity of the light from injuring the optick nerve. 
Observe Emily's eyes, as she sits looking towards the win- 
dows ; her pupils appear very small, and the iris large. 
Now, Emily, turn from the light and cover your eyes with 
your hand, so as entirely to exclude it for a few moments. 

Caroline. How very much the pupils of her eyes are 
now enlarged, and the iris diminished. This is, no doubt, 
the reason why the eyes suffer pain, when from darkness 
they suddenly come into a strong light ; for the pupil be- 
ing dilated, a quantity of rays must rush in before it has 
time to contract. 

Emily. And when we go from a strong light into ob- 
scurity, we at first imagine ourselves in total darkness ; 
for a sufficient number of rays cannot gain admittance 
into the contracted pupil, to enable us to distinguish ob- 
jects : but in a few minutes it dilates, and we clearly per- 
ceive objects which were before invisible. 

Mrs. B. It is just so. The choroid c c, is imbued 
with a black liquor which serves to absorb all the rays 
that are irregularly reflected, and to convert the body of 
the eye into a more perfect camera obscura. When the 
pupil is expanded to its utmost extent, it is capable of ad- 
mitting ten times the quantity of light that it does when 
most contracted. In cats, and animals which are said to 
see in the dark, the power of dilatation and contraction 
of the pupil is still greater ; it is computed that their pu- 
pils may receive one hundred times more light at one 
time than at another. 

Within these coverings of the eye-ball are contained 
three transparent substances, called humours. The first 



971. When is it dilated, and when contracted ? 972. Why 

does it give the eyes pain on first going into a* bright light from a 
dark room ? 973. Why does it seem much darker on first go- 
ing out in the night, than after we have been out a short time ? 

974. How much more light is admitted when the pupil is 

extended to the utmost, than when most contracted? 975. 

Why can cats, horses, and some other animals, see better in the 

night than we can ? 976. How much is it thought the pupil 

of their eyes extend and contract ? 977. What is contained 

within the coverings of the eye-ball ? 



OPTICKS. 231 

occupies the space immediately behind the cornea, and is 
called the aqueous humour, ff, from its liquidity and its 
resemblance to water. Beyond this is situated ihe crys- 
talline humour, g g, so called from its clearness and trans- 
parency : it has the form of a lens, and refracts the rays of 
light in a greater degree of perfection than any that have 
been constructed by art : it is attached by two muscles, 
m m, to each side of the choroid. The back part of the 
eye, between the crystalline humour and the retina, is fill- 
ed by the vitreous humour, h A, which derives its name 
from a resemblance it is supposed to bear to glass or vi- 
trified substances. 

The membraneous coverings of the eye are intended 
chiefly for the preservation of the retina, i i, which is by 
far the most important part of the eye, as it is that which 
receives the impression of the objects of sight, and con- 
veys it to the mind. The retina consists of an expansion 
of the optick nerve, of a most perfect whiteness : it pro- 
ceeds from the brain, enters the eye, at n, on the side next 
the nose, and is finely spread over the interiour surface 
of the choroid. 

The rays of light which enter the eye by the pupil are 
refracted by the several humours in their passage through 
them, and unite in a focus on the retina. 

Caroline. I do not understand the use of these refract- 
ing humours ; the image of objects is represented in the 
eamera obscura, without any such assistance. 

Mrs. B. That is true ; but the representation would 
be much more strong and distinct, if we enlarge the open- 
ing of the camera obscura, and received the rays into it 
through a lens. 

I have told you that rays proceed from bodies in all 
possible directions. We must, therefore, consider every 
part of an object which sends rays to our eyes, as points 
from which the rays diverge, as from a centre. 

973. What are the three humours called ? 979. From 

what does the aqueous humour derive its name ? 980. From 

what does the crystalline humour derive its name ? 981. From 

what does the vitreous humour derive its name ? 982. For what 

are the membraneous coverings of the eye chiefly intended ? 

983. Which part of the figure exhibits the retina ? 984. Of 

what does the retina consist ? 985. How is the light which 

enters the pupil affected by the several humours ? 986. What 

would be the consequence if the light admitted by the pupil were 
not refracted by the humours ? 



232 opticus. 

Emily. These divergent rays, issuing from a single 
point, I believe you told us, were called a pencil of rays 1 

Mrs. B. Yes. Now, divergent rays, on entering the 
pupil, do not cross each other ; the pupil, however, is 
sufficiently large to admit a small pencil of them ; and 
these, if not refracted to a focus by the humours, would 
continue diverging after they had passed the pupil, would 
fall dispersed upon the retina, and thus the image of a 
single point would be expanded over a large portion of 
the retina. The divergent rays from every other point 
of the object would be spread over a similar extent of 
space, and would interfere and be confounded with the 
first ; so that no distinct image could be formed, and the 
retina would represent total confusion both of figure and 
colour. Fig. 3 represents two pencils of rays issuing 
from two points of the tree A B, and entering the pupil C, 
refracted by the crystalline humour D, and forming dis- 
tinct images of the spot they proceed from, on the retina 
at a b. Fig. 4 differs from the preceding, merely from 
not being supplied with a lens ; in consequence of which 
the pencils of rays are not refracted to a focus, and no 
distinct image is formed on the retina. I have delineated 
only the rays issuing from two points of an object, and 
distinguished the two pencils in fig. 4, by describing one 
of them with dotted lines ; the interference of these two 
pencils of rays on the retina will enable you to form an 
idea of the confusion which would arise, from thousands 
and millions of points at the same instant pouring their 
divergent rays upon the retina. 

Emily. True ; but I do not yet well understand how 
the refracting humours remedy this imperfection. 

Mrs. B. The refraction of these several humours 
unite the whole of a pencil of rays, proceeding from any 
one point of an object, to a corresponding point on the 
retina, and the image is thus rendered distinct and strong. 
If you conceive, in fig. 3, every point of the tree to send 
forth a pencil of rays similar to those, A B, every part of 
the tree will be as accurately represented on the retina as 
the points a b. 

Emily. How admirably, how wonderfully, this is con- 
trived ! 

987. What does Fig. 3. plate XXI. represent ? 988. What 

does Fig. 4. of that plate represent? 989. How does the re- 
fracting humour remedy the defects exhibited in that figure ? 



opticks. 233 

Caroline. But since the eye requires refracting hu- 
mours in order to have a distinct representation formed 
on the retina, why is not the same refraction necessary 
for the image formed in the camera obscura ? 

Mrs. B. Because the aperture through which we re- 
ceived the rays into the camera obscura is extremely 
small ; so that but very few of the rays diverging from a 
point, gain admittance ; but we will now enlarge the 
aperture, and furnish it with a lens, and you will find the 
'andscape to be more perfectly represented. 
Caroline. How obscure and confused the image is now that 
you have enlarged the opening, without putting in the lens ! 

Mrs. B. Such or very similar would be the representa- 
tion on the retina, unassisted by the refracting humours. 
But see what a difference is produced by the introduction 
of the lens, which collects each pencil of divergent rays 
into their several foci. 

Caroline. The alteration is wonderful : the represen- 
tation is more clear, vivid, and beautiful than ever. 

Mrs. B. You will now be able to understand the na- 
ture of that imperfection of sight, which arises from the eyes 
being too prominent. In such cases, the crystalline humour, 
D, (fig. 5.) being extremely convex, refracts the rays too 
much, and collects a pencil, proceeding from the object A 
B, into a focus, F, before they reach the retina. From this 
focus the rays proceed diverging, and consequently form a 
very confused image on the retina at a b. This is the de- 
fect of short-sighted people 1 

Emily. I understand it perfectly. But why is this 
defect remedied by bringing the object nearer to the eye, 
as we find to be the case with short-sighted people 1 

Mrs. B. The nearer you bring an object to your eye, 
the more divergent the rays fall upon the crystalline hu- 
mour, and they are consequently not so soon converged to 
a focus ; this focus, therefore, either falls upon the retina, 
or at least approaches nearer to it, and the object is pro- 
portionally distinct, as in fig. 6. 

Emily. The nearer, then, you bring an object to a 
lens, the further the image recedes behind it. 

990. Why is not something like the refracting humours neces- 
sary in the camera obscura ? 991. What peculiarity of the eye 

causes some persons to be short-sighted ? 992. Which figure 

represents the eye of a short-sighted person ? 993. Why can 

short-sighted persons see better by bringing the objects near to 

the eye ? 994. By which figure is this illustrated : 

20 ■ 



234 opticks. 

Mrs. B. Certainly. But short-sighted persons have 
another resource for objects which they cannot approach 
to their eyes ; this is to place a concave lens, C D, (fig. 
1, plate XXII.) before the eye, in order to increase the di- 
vergence of the rays. The effect of a concave lens is, 
you know, exactly the reverse of a convex one : it renders 
parallel rays divergent, and those which are already diver- 
gent, still more so. By the assistance of such glasses, 
therefore, the rays from a distant object fall on the pupil, 
as divergent as those from a less distant object ; and, 
with short-sighted people, they throw the image of a dis- 
tant object back as far as the retina. 

Caroline. This is an excellent contrivance, indeed. 

Mrs. B. And tell me, what remedy would you devise 
for such persons as have a contrary defect in their sight ; 
that is to say, in whom the crystalline humour, being too 
flat, does not refract the rays sufficiently, so that they 
reach the retina before they are converged to a point ? 

Caroline. I suppose that a contrary remedy must be 
applied to this defect ; that is to say, a convex lens, L M, 
fig. 2. to make up for the deficiency of convexity of the 
crystalline humour O P. For the convex lens would 
bring the rays nearer together, so that they would fall 
either less divergent, or parallel on the crystalline humour ; 
and, by being sooner converged to a focus, would fall on 
the retina. 

Mrs. B. Very well, Caroline. This is the reason 
why elderly people, the humours of whose eyes are decay- 
ed by age, are under the necessity of using convex specta- 
cles. And when deprived of that resource, they hold the 
object at a distance from their eyes, as in fig. 4., in order 
to bring the focus forwarder. 

Caroline. I have often been surprised, when my 
grandfather reads without his spectacles, to see him hold 
the book at a considerable distance from his eyes. But I 
fiow understand it ; for the more distant the object is 
from the crystalline, the nearer the image will be to it. 

995. What other resource have short-sighted persons, for reme- 
dying the defect of their eyes ? 996. Why will a concave lens 

remedy this defect ? 997. What is the design of Fig. 1, plate 

XXII. P 998. What is the reason that elderly people usually 

lose their sight ? 999. What remedy is there for the eyes when 

the humours are decayed or flattened ? 1000. Which figure 

illustrates this? 1001. Why do old people without convex 

glasses hold the objects to be seen at a distance from the eye ? 



opticks. 235 

Emily. I comprehend the nature of these two oppo- 
site defects very well ; but I cannot now conceive, how 
any sight can be perfect : for if the crystalline humour is 
of a proper degree of convexity, to bring the image of dis- 
tant objects to a focus on the retina, it will not represent 
near objects distinctly ; and if, on the contrary, it is adapt- 
ed to give a clear image of near objects, it will produce 
a very imperfect one of distant objects. 

Mrs. B. Your observation is very good, Emily : and 
it is true, that every person would be subject to one of 
these two defects, if we had it not in our power to in- 
crease or diminish the convexity of the crystalline humour, 
and to project it towards, or draw it back from the object, 
as circumstances require. In a young well constructed 
eye, the two muscles to which the crystalline humour is 
attached, have so perfect a command over it, that the focus 
of the rays constantly falls on the retina, and an image is 
formed equally distinct both of distant objects, and of 
those which are near. 

Caroline. In the eyes of fishes, which are the only 
eyes I have ever seen separate from the head, the cornea 
does not protrude, in that part of the eye which is exposed 
to view. 

Mrs. B. The cornea of the eye of a fish is not more 
convex than the rest of the ball of the eye ; but to supply 
this deficiency, their crystalline humour is spherical, and 
refracts the rays so much, that it does not require the as- 
sistance of the cornea to bring them to a focus on the re- 
tina. 

Emily. Pray, what is the reason that we cannot see 
an object distinctly, if we approach it very near to the eye ? 

Mrs. B. Because the rays fall on the crystalline hu- 
mour too divergent to be refracted to a focus on the retina ; 
the confusion, therefore, arising from viewing an object 
too near the eye, is similar to that which proceeds from a 
flattened crystalline humour : the rays reach the retina be- 
fore they are collected to a focus, (fig. 4.) If it were not 
for this imperfection, we should be able to see and distin- 

1002. By what means can the same eye see distinctly distant 

objects and those which are near ? 1003. What peculiarity of 

structure is there in the eyes of fishes ? 1004. How is this 

seeming defect remedied ? 1005. What is the reason that we 

■cannot see an object distinctly when it is placed very near to the 
eye ? 100fi. By which figure is this illustrated ? 



236 0PTICKS. 

guish the parts of objects, which are now invisible to us, 
from their minuteness ; for could we approach them very 
near the eye, their image on the retina would be so much 
magnified as to render them visible. 

Emily. And could there be no contrivance to convey 
the rays of objects viewed close to the eye, so that they 
should be refracted to a focus on the retina. 

Mrs. B. The microscope is constructed for this pur- 
pose. The single microscope (fig. 5.) consists simply of 
a convex lens, commonly called a magnifying-glass ; in 
the focus of which the object is placed, and through which 
it is viewed : by this means you are enabled to approach 
your eye very near the object, for the lens, A B, by di- 
minishing the divergence of the rays, before they enter 
the pupil C, makes them fall parallel on the crystalline 
humour D, by which they are refracted to a focus on the 
retina, at R R. 

Emily. This is a most admirable invention, and no- 
thing can be more simple, for the lens magnifies the ob- 
ject merely by allowing us to bring it nearer to the eye. 

Mrs. B. Those lenses, therefore, which have the 
shortest focus, will magnify the object most, because they 
enable us to bring the object nearest to the eye. 

Emily. But a lens, that has the shortest focus, is most 
bulging or convex ; and the protuberance of the lens will 
prevent the eye from approaching very near to the object. 

Mrs. B. This is remedied by making the lens ex- 
tremely small : it may then be spherical without occupy- 
ing much space, and thus unite the advantages of a short 
focus, and of allowing the eye to approach the object. 

Caroline. We have a microscope at home, which is a 
much more complicated instrument than that you have 
described. 

Mrs. B. It is a double microscope, (fig. 6.) in which 
you see not the object A B, but a magnified image of it, 
ah. In this microscope, two lenses are employed, the 
one L M, for the purpose of magnifying the object, is 

1007. In what way can objects be seen distinctly when placed 
near the eye ? 1008. Of what does a single microscope con- 
sist ? 1009. What is the object of Fig. 5, plate XXII. ? 

1010. What lenses will magnify objects most ? 1011. What 

kind of lenses has the shortest focus ? 101& What is repre- 
sented by Fig. fi, plate XXII. ? 1013. How would you explain 

the use of the double mi< Toscope, by the aid of that figure ? 



opticks. 237 

called the object-glass ; the other N O, acts on the prin- 
ciple of the single microscope, and is called the eye-glass. 

There is another kind of microscope, called the solar 
microscope, which is the most wonderful from its great 
magnifying power ; in this we also view an image formed 
by a lens, not the object itself. As the sun shines, I can 
show you the effect of this microscope : but for this pur- 
pose, we must close the shutters, and admit only a small 
portion of light, through the hole in the window-shutter, 
which we used for the camera obscura. We shall now 
place the object A B, (plate XXIII. fig. 1.) which is a 
small insect, before the lens, C D, and nearly at its fo- 
cus ; the image E F, will then be represented on the op- 
posite wall in the same manner as the landscape was in the 
camera obscura ; with this difference, that it will be mag- 
nified, instead of being diminished. I shall leave you to 
account for this, by examining the figure. 

Emily. I see it at once. The image E F is magnified, 
because it is further from the lens, than the object A B ; 
while the representation of the landscape was diminished 
because it was nearer the lens, than the landscape was, 
A lens, then, answers the purpose equally well, either for 
magnifying or diminishing objects ? 

Mrs. B. Yes ; if you wish to magnify the image,you 
place the object near the focus of the lens ; if you wish 
to produce a diminished image, you place the object at a 
distance from the lens, in order that the image may be 
formed in or near the focus. 

Caroline. The magnifying power of this microscope 
is prodigious, but the indistinctness of the image for want 
of light, is a great imperfection. Would it not be clearer, 
if the opening in the shutter were enlarged, so as to ad- 
mit more light 1 

Mrs. B. If the whole of the light admitted does not 
fall upon the object, the effect will only be to make the 
room lighter, and the image consequently less distinct. 

Emily. But could you not by means of another lens 
bring a large pencil of rays to a focus on the object, and 
thus concentrate the whole of the light admitted upon it 1 

1014. What does Fig. 1, plate XXIII. represent? 1015. How 

would you describe a solar microscope b?r the use of this figure ? 

1016. Where must an object be placed in regard to a lens, 

so that the object be magnified ? 1017. Where, so that the ob- 
ject be diminished ? 1018. Where must all the light fall, used 

in the solar microscope, so that the effect be the most favourable -t 



238 opticks. 

Mrs'. B. Very well. We shall enlarge the opening 
and place the lens X Y (fig. 2.) in it, to converge the rays 
to a focus on the object A B. There is but one thing 
more wanting to complete the solar microscope, which I 
shall leave to Caroline's sagacity to discover. 

Caroline, Our microscope has a small mirror attached 
to it, upon a moveable joint, which can be so adjusted as 
to receive the sun's rays, and reflect them upon the ob- 
ject ; if a similar mirror were placed to reflect light upon 
the lens, would it not be a means of illuminating the ob- 
ject more perfectly 1 

Mrs. B. You are quite right. CI f, (fig. 2.) is a 
small mirror placed on the outside of the window-shutter, 
which receives the incident rays S S, and reflects them on 
the lens X Y. New that we have completed the appara- 
tus, let us examine the mites on this piece of cheese, 
which I place near the focus of the lens. 

Caroline. Oh ! how much more distinct the image 
now is, and how wonderfully magnified ; the mites on 
the cheese look like a drove of pigs scrambling over rocks. 

Emily. I never saw any thing so curious. Now an 
immense piece of cheese has fallen : one would imagine 
it an earthquake : some of the poor mites must have been 
crushed ; how fast they run, — they absolutely seem to 
gallop. 

But this microscope can be used only for transparent 
objects ; as the light must pass through them to form the 
image on the wall. 

Mrs. B. Very minute objects, such as are viewed in 
a microscope, are generally transparent; but when opaque 
objects are to be exhibited, a mirror M N (fig. 3. ) is used 
to reflect the light on the side of the object next the wall : 
the image is then formed by light reflected from the object, 
instead of being transmitted through it. 

Emily. Pray is not a magick lantern constructed on 
the same principles !• 

* The magick lantern is an instrument used for magnifying 
paintings on glass, and throwing their images upon a white screen 
in a darkened chamber. 

1019. What does fig. 2, plate XXIII. represent ? 1020. What 

is the use of the mirror in the solar microscope ? 1021. For 

what objects can the solar microscope be used ? 1022. How 

can opaque objects be exhibited ? 1023. Which figure illus- 
trates this ? 1024. What is a magick lantern ? 



ofxicKs. 239 

Mrs. B. Yes ; with this difference, that the light is 
supplied by a lamp, instead of the sun. 

The microscope is an excellent invention, to enable us 
to see and distinguish objects, which are too small to be 
visible to the naked eye. But there are objects which, 
though not really small, appear so to us, from their dis- 
tance ; to these we cannot apply the same remedy ; for 
when a house is so far distant, as to be seen under the same 
angle as a mite, which is close to us, the effect produced 
on the retina is the same : the angle it subtends is not 
large enough for it to form a distinct image on the retina. 

Emily. Since it is impossible, in this case, to approach 
the object to the eye, cannot we by means of a lens bring 
an image of it nearer to us ? 

Mrs. B. Yes ; but then the object being very distant 
from the focus of the lens, the image would be too small 
to be visible to the naked eye. 

Emily. Then, why not look at the image through ano- 
ther lens, which will act as a microscope, enable us to 
bring the image close to the eye, and thus render it visi- 
ble ? 

Mrs. B. Very well, Emily ; I congratulate you on 
having invented a telescope. In figure 4, the lens C D, 
forms an image E F, of the object A B ; and the lens X 
Y, serves the purpose of magnifying that image ; and this 
is all that is required in a common refracting telescope. 

Emily. But in fig. 4, the image is not inverted on the 
retina, as objects usually are : it should therefore appear 
to us inverted ; and that is not the case in the telescopes I 
have looked through. 

Mrs. B. When it is necessary to represent the image 
erect, two other lenses are required ; by which means a 
second image is formed, the reverse of the first and conse- 
quently upright. These additional glasses are used to 
view terrestrial objects ; for no inconvenience arises from 
seeing the celestial bodies inverted. 



1025. How does a magick lantern differ from a solar micro- 
scope? 1026. What is the reason that the solar microscope may 

not be used with objects at a great distance with equal effect ? 

1027. What does Fig. 4, plate XXIII. represent ? 1028. How 

would you explain the principle of the common refracting tele- 
scope by the use of that figure ? 1029. What is necessary when 

the image of an object is to be exhibited erect ? 1030. Why are 

not these additional glasses used in viewing celestial objects ? 



240 OPTICKS. 

Emily. The difference between a microscope and a - 
telescope seems to be this : — a microscope produces a mag- 
nified image, because the object is nearest the lens ; and 
a telescope produces a diminished image, because the ob- 
jects farthest from the lens. 

Mrs. B. Your observation applies only to the lens C 
D, or object glass, which serves to bring an image of the 
object nearer the eye ; for the lens X Y, or eye-glass, is in 
fact a microscope, as its purpose is to magnify the image. 

When a very great magnifying power is required, tele- 
scopes are constructed with concave mirrors, instead of 
lenses. Concave mirrors, you know, produce, by reflec- 
tion, an effect similar to that of convex lenses by refrac- 
tion. In reflecting telescopes, therefore, mirrors are used 
in order to bring the image nearer the eye ; and a lens or 
eye-glass the same as in the refracting telescope to mag- 
nify the image. 

The advantage of the reflecting telescope, is, that mir- 
rors whose focus is six feet, will magnify as much as lenses 
of a hundred feet. 

Caroline. But I thought it was the eye-glass only which 
magnified the image ; and that the other lens served to 
bring a diminished image nearer to the eye. 

Mrs. B. The image is diminished in comparison to 
the object, it is true ; but it is magnified if you compare 
it to the dimensions of which it would appear without the 
intervention of any opjical instrument ; and this magni- 
fying power is greater in reflecting than in refracting 
telescopes. 

We must now bring our observations to a conclusion, 
for I have communicated to you the whole of my very 
limited stock of knowledge of Natural Philosophy. If it 
will enable you to make further progress in that science, 
my wishes will be satisfied; but remember that, in order 
that the study of nature may be productive of happiness, 
it must lead to an entire confidence in the wisdom and 
goodness of its bounteous Author. 

1031. What part of the telescope exhibited in the figure may 

be considered as a simple microscope ? 1032. When a very 

great magnifying power is required, how must telescopes be con- 
structed ? 1033. In the reflecting telescopes why are mirrors 

usod ? 1034. How great is the advantage of the reflecting 

telescope ? 



A DICTIONARY OF PHILOSOPHICAL TERMS. 



ABERRATION, in astronomy, an appa- 
rent motion of the heavenly bodies, pro- 
duced by the progressive motion of light 
and the earth's annual motion. 

ACCELERATION, in mechanick's, de- 
notes the augmentation or increase of mo 
tion in accelerated bodies. 
ACOUSTICKS is the science which treats 
of the nature, phenomena, and laws of the 
sense of sound. It extends to the theory of 
musical concord and harmony, and is, there- 
fore, a valuable and interesting science. 
AIR, a thin, elastick fluid, surrounding the 
globe of the earth. The air, together with 
the clouds and vapours that float in it, is 
called the atmosphere. The height to which 
I the atmosphere extends has never been as- 
1 certained ; but, at a greater height than 45 
| miles, it ceases to reflect the rays of light 
i from the sun. 

AIR-PUMPS are machines made for ex- 
hausting the air from certain glass vessels, 
adapted to the purpose of experiments on 
air. 

ANGLE is the inclination of two lines 
meeting one another in a point, and called 
the legs of the angle. Angles, in geometry, 
are called right, acute, and obtuse. A right 
angle contains just 90 degrees, or the quar- 
ter part of a circle. Acute angles contain 
less,and obtuse angles more than 90 degrees. 

ANGLE OF INCIDENCE is that which 
is contained between the line described by 
the incident ray, and a line perpendicular 
to the surface on which the ray strikes, 
raised from the point of incidence. 

ANGLE OF REFLECTION is contain- 
ed between the line described by the re- 
flected ray, and a line perpendicular to the 
reflecting surface, at the point from which 
the ray is reflected. 

ANGLE OF REFRACTION is that 
which is contained between the line describ- 
ed by the refracted ray, and a line perpendi 
cular to the refracting surface at the point 
in which the ray passes through that sur- 

ANGLE OF VISION is that which is 
contained between lines coming from oppo- 
I ( site parts of an object and meeting in the 
eye. 

I ANT ARCTICK CIRCLE, in astronomy, 
is an imaginary line extending round the 
I (south pole, 66 1-2 degrees from the equator 
|| and parallel to it. 

I (> APHELION, in astronomy, is that point 
in any planet's orbit in which the orbit is 
nost distant from the sun. 
AQUEOUS HUMOUR, or watery hu- 
lour of the eye ; it is the first and outermost, 
nd that which is less dense than either 
the vitreous or crystalline. It is transpa- 
rent and colourless like water, and fills up 
|the space that lies between the cornea and 
the crystalline humour. 

ARCTICK CIRCLE, in astronomy, is an 
f imaginary line extending round the north 

21 



pole, 66 1-2 degrees from the equator and 
parallel to it. 

AREOMETER, an instrument by which 
the density and gravity of fluids are mea- 
sured. 

ARIES, in astronomy, a constellation of 
fixed stars, drawn on the globe in the figure 
of a ram. It is the first of the twelve signa 
of the zodiac from which a twelfth part of 
the ecliptick takes its name. It consists of 
sixty-six stars. 

ASCENSION, in astronomy, the rising 
of the sun or star, or any part of the equi- 
noctial with it, above the horizon. 

ASTERIODS, a name given by Dr. Her- 
schel to the new planets, Ceres, Juno, Pal- 
las, and Vesta, lately discovered. 

ASTRONOMY is the science which 
teaches the motions of the earth, the sun, 
moon, planets, comets, and stars, and ex- 
plains the phenomena occasioned by those 
motions. 

ATMOSPHERE, or atmospherick air, the 
fluid that surrounds our earth. Without 
this fluid no animal could exist ; vegetation 
would cease, and there would be neither 
rain nor refreshing dews to moisten the 
face of the ground ; and though the sun and 
stars might be seen as bright specks, yet 
there would be little enjoyment of light, 
could we exist without it. 

ATTRACTION, a general term, used to 
denote the power or principle by which bo- 
dies mutually tend towards each other, 
without regarding the cause or action that 
may be the means of producing the effect. 

ATTRACTION OF COHESION takes 
place between the constituent particles of 
the same body. By this principle bodies 
preserve their forms and are prevented from 
falling to pieces. 

ATTRACTION OF GRAVITATION, 
or gravity, is the name of that force by 
which distant bodies tend towards one 
another. 

AXIS of a planet is an imaginary line 
which passes through its centre, and on 
which it turns ; and it is this motion which 
produces day and night. With that side of 
the planet facing the sun it is day ; and 
with the opposite side, which remains in 
darkness, it is night. 
AXIS OF MOTION, in mechanicks, is the 
line about which a revolving body moves. 
Philosophically speaking, the axis of mo- 
tion is said to be at rest, whilst the other 
parts of a body move round it; and the 
further any part of a body is from the axis 
of motion, the greater is its velocity. 

AXIS OF THE EARTH is an imagina- 
ry line conceived to pass through the centre 
of it from one pole to the other, about which 
is performed its diurnal rotation. 

AXIS, in opticks, is that ray, among all 
others that are sent to the eye, which falls 
perpendicularly upon it, and which conse- 
quently passes through the centre of the eye. 



34S 



A DICTIONARY OF 



AXIS OF A GLASS, OR LENS, is a 
right line joining the middle points of the 
two opposite surfaces of the glass. 

FAL ANCE, or BALLANCE, in mecha- 
nicks, one of the simple powers which serves 
to find out the equality or difference of 
weight in heavy bodies. 

BALLOON, "a machine used in naviga- 
tion through the air. It takes its name 
from the form of the machine, the word 
balloon signifying any spherical hollow bo- 
dy, of whatever matter it be composed, or 
for whatever purposes it be designed. 

BAROMETER, an instrument for mea- 
suring the weight or pressure of the at- 
mosphere ; and by that means measuring 
heights and depths, determining variations 
in the state of the air, and foretelling the 
changes in the weather. 

BASE, in geometry, the lowest side of 
the perimeter of a figure. Thus, the base 
of a triangle may be said of any of its sides, 
but more properly of the lowest, or that 
which is parallel to the horizon. In rec- 
tangled triangles, the base is properly that 
side opposite to the right angle. 

BASS, in musick, that part of a concert 
which is most heard, which consists of the 
gravest and deepest sounds, and which is 
played on the largest pipes or strings of the 
instrument. 

BODY, in physicks, an extended solid sub- 
stance, of itself utterly passive and inactive, 
indifferent either to motion or rest : but ca- 
pable of any sort of motion, and of all figures 
and forms. Body, or substance, which is 
the same thing, is usually denoted by the 
general term matter. 

BREADTH, in geometry, one of the 
three dimensions of bodies, which, multi- 
plied into their length, constitutes a sur- 
face. 

BUBBLE, in philosophy, small drops or 
vesicles of any fluid filled with air, and 
either formed on its surface, by an addition 
of more of the fluid, or in its substance, by 
an intestine motion of its component parts. 

BURNING-GL AvSS, a convex or concave 
glass, commonly spherical, which, being 
exposed directly to the sun, collects all the 
rays falling thereon into a very small space, 
called the foens, where wood, or any other 
combustible substance, bein£ put, will be 
set on fire. 

CAMERA-OBSCURA, inopticks, a ma- 
chine representing an artificial eye. It is 
made by placing a convex glass in a hole 
of a window shutter, and if no light enters 
the room but through the glass, the pictures 
of all objects on the outside may be distinct- 
ly seen in an inverted position, on any 
white surface placed at the focus of the 
lens. 

PILLAR Y TUBES, in physicks. little 
pipes, whose canals are extremely narrow, 



that point about which all the parts of ft 
body do, in any situation, balance each 
other. 

CENTRE OF MOTION, that point 
which remains at rest, while all the other 
parts of a body move about it. 

CENTRAL FORCES, the powers which 
cause a moving body to tend towards, or 
recede from, the centre of motion. 

CENTRIFUGAL FORCE, that by 
which ail bodies, that move round any 
other body in a curve, endeavour to fly off 
from the axis of their motion in a tangent. 

CENTRIPETAL FORCE, that force by 
which a body is every where impelled, or 
any how tends towards some point as a 
centre ; such as gravity, or that force 
whereby bodies tend towards the centre of 
the earth ; magnetical attraction, whereby 
the loadstone draws iron ; and that force, 
whatever it be, whereby the planets are 
continually drawn back from right-lined 
motions, and made to move in curves. 

CHROMATICKS is that part of opticks 
which explains the several properties of 
the colours of light and of natural bodies. 

CIRCLE, in geometry, a plane figure 
comprehended by a single curve line, called 
its circumference, to which right lines or 
radii, drawn from a point in the middle, 
called the centre, are equal to each other. 

CIRCUMFERENCE, in a general sense, 
denotes the line or lines bounding a plane 
figure. However, it is generally used in a 
more limited sense, for the curve line which 
bounds a circle, and otherwise called a pe- 
riphery, the boundary of a right lined 
figure being expressed by the term perime- 
ter. 

CLOUDS are a collection of misty va- 
pours suspended in the air. Their various 
colours and appearances are owing to their 
particular situation in regard to the sun, 
to the different reflection of the sun's rays, 
and to the effects produced on them by 
wind. 

COHESION, one of the species of attrac- 
tion, denoting that force by which the parte 
of bodies stick together. 

COLOUR mean3 that property of bodies 
which affects the sight only ; thus the grass 
in the fields has a green colour, blood has a 
red colour, the sky generally appears of a 
blue colour, and thus of others that might 
be named. The variety of colours, as they 
are presented to us by the substances that 
surround us, is immense, and from them- 
arises the admirable beauty of the works 
of nature in the animal, in the vegetable, 
and in the mineral kingdom, or, more pro- 
per Iv speaking, in the universe. 

COLURES, in astronomy and geogra- 
phy, two great circles, supposed to inter- 
sect each other at right angles in the poles 
of the world, and to pass through the sol- 



user! for experiments in illustrating cohe- stitial and equinoctial points of the eclip 
five attraction. tick. 

< WIUCORN, in astronomy, one of the) COMETS are opaque and solid bodies. 

iyna of the zodiack, represented on j A comet, at a given distance from the earth. 

p lobes in the form of a goat. -bines mucti brighter when it is on the 

rENTREOFGRAVlTYJnmechanickdJsamesideof the earth with the sun tb*r> 



PHILOSOPHICAL TERMS- 



243 



when it is on the contrary side ; from ing to make that refraction of the rays of 
whence it appears that it owes it3 bright- light, necessary to make them meet in the 
ness to the sun. retina, and form an image thereon, where- 

COMPLE.UENT, in astronomy, the dis- by vision may be performed, 
tance of a st-T from the zenith ; or the arch CYLINDER, in geometry, a solid body, 
comprehended between the place of the supposed to be generated by the rotation 
star above the horizon and the zenith of a parallelogram. 

COMPRESSION, the act of pressing or DAY. In common language, the day is 
squeezing some matter, so as to set its trie interval of time which elapses from the 
parts nearer to each other, and make it rising to the setting of the sun. The as t re- 
possess less space. nomical day embraces the whole interval 

CONCAVE, an appellation used in which passes during a complete revolution 
speaking of the inner surface of hollow bo- of the sun. 
dies', but more especially of spherical ones. DECLINATION, in astronomy, the dis- 

CONCORD, in musick, the relation of two tance of any celestial object from the equi- 
sounds that are always agreeable to the noctial, either northward or southward. It 
ear, whether applied in succession or con- is either true or apparent, according as the 
sonance. real or apparent place of the object is con- 

CONDENSER, a pneumatick engine or sidered. 
■yringe, whereby an uncommon quantity DEGREE, in geometry, a division of a 
of air may be crowded into a given space ; circle including a three hundred and six- 
so that sometimes ten times as much air as tieth part of its circumference. Every 
there is at the same time in the same space, circle is supposed to be divided into three 
without the engine, may be thrown in by hundred and sixty parts, called degrees, 
means of it, and its egress prevented by aad each degree divided into sixty other 
valves properly disposed. parts called minutes ; and each of these 

CONDUCTORS, in electricity, are long minutes is again divided into sixty seconds, 
metal rods, whose points are raised above DENSITY denotes the degree of close- 
the buildings to which the conductors are ness and compactness of the particles of a 
affixed, for the purpose of attracting or re- body ; and is that property directly oppo* 
ceiving the electrick fluid, and of conduct- site to rarity. 

ing it into the earth, or into water, thereby, DEPRESSION OF THE POLE. "When 
to prevent such buildings from being struck a person sails or travels towards the eqna- 
by lightning. tor, he is said to depress the pole, because 

CONE, in geometry, a solid figure, hav- as many degrees as he approaches nearer 
ing a circle for its base, and its top termi- the equator, so many degrees will the pole 
nated in a point or vertex. be nearer the horizon. The phenomenon 

CONJUNCTION, in astronomy, is the arises from the spherical figure of the earth, 
meeting of two stars or planets in "the same; DIAGONAL, in geometry, a right line 
degree of the zodiack. drawn across a quadrilateral figure, from 

CONSTELLATION, in astronomy, a one angle to another, by some called the 
system of several stars that are seen in the diameter of the figure, 
heavens near to one another. Astronomers DIAMETER, in geometry, a right lina 
not only mark out the stars, but they dis- passing through the centre of a circle, and 
tribute them into asterisms, or constella- terminated at each side by the circumfV 
tions, allowing several stars to make up rence thereof! 

one constellation : — and for the better dis-! DIGIT, in astronomy, the twelfth part 
tinguishing and observing them, they re- of the diameter of the sun or moon, is used 
duce the constellations to the forms of ob- to express the quantity of an eclipse. Thuf 
jects with which we are well acquainted. Ian eclipse is said to be six digits, when six 

CONVERGING, or convergent lines, in of these parts are hid. 
geometry, are such as continually approach DIMENSION, in geometry, is either 
nearer one another ; or whose distance be- breadth, length, or thickness ; hence a line 
comes still less and less. has one dimension, viz. length; a superfi- 

CONVERGING RAYS, in opticks, are cies, two, fit. length and breadth; and a 
those rays, that, issuing from diverse points body, or solid, has three, to wit, length, 
of an object, incline towards one another .breadth, and thickness. 
till, at last, they meet and cross, and then DIRECTION, in mechanicks, signifies the 
become diverging rays. line or path of a body's motion, along which 

CONVEX, an appellation given to the it endeavours to proceed, according to the 
exteriour surface of gibbous or globular bo- force impressed upon it. 
dies, in opposition to the hollow inner sur-! DISK, in astronomy, the body and face 
face of such bodies, which is called concave. of the sun and moon, such as it appears to 
Thus we say a convex lens, a convex mir-ius on the earth, or the body or face of the 
ror, and convex superficies. 'earth, such as it appears to a spectator in 

CORNEA, the second coat of the eye, so'the moon. The disk in eclipses is supposed 
called from its substance, which resemblesito be divided into twelve equal parts, 
the horn of a lantern. | DISCORD, in musick, a dissonant and up- 

CRYSTALLINE HUMOUR,a thick com- harmonious combination of sounds, so call- 
pact humour, in form of a flattish convex ed in opposition to concord, 
lens, situated in the middle of the eve, serv-' DIVERGENT BAYS, in opticks, ua 



244 



A DICTIONARY OF 



those, which, going from a point of the vi 
Bible object, are dispersed, and continually 
depart one from another, in proportion as 
they are removed from the object ; in which 
sense it is opposed to convergent. 

DIVISIBILITY, that property by which 
the particles of matter in all bodies are ca- 
pable of a separation, or disunion from 
each other. 

DIURNAL, in astronomy, something re- 
lating to the day, in opposition to noctur- 
nal, which regards the night. The diurnal 
motion of a planet, is so many degrees and 
minutes as any planet moves in twenty- 
four hours. Hence the motion of the earth 
about its axis is called its diurnal motion. 

DROPS, in meteorology, small spherical 
bodies, into which the particles of fluids 
spontaneously form themselves, when let 
fall from any height. 

DUCT denotes any tube or canal. *• 

DUCTILITY, in physicks, a property of 
certain bodies, whereby they are capable of 
being expanded, or stretched forth by means 
of a hammer or press. 

DYNAMICKS. This branch of mecha 
nicks relates to the action of forces that give 
motion to solid bodies ; which forces are cal- 
culated, both by their active powers, and 
by the proportion cf time in which those 
powers become efficient. 

EARTH, the vast mass or planet which 
we inhabit. The ancients supposed the 
earth flat or cylindrical ; but from the ge- 
neral appearance of the planetary system. 
from the circular shadow of the earth in 
eclipses of the moon, and from the fact that 
the earth has been circumnavigated, it is 
concluded by the moderns, that it is sphe- 
rical. 

EARTHQUAKE is a sudden motion of 
the earth, occasioned, it is supposed, either 
by the discharge of some electrical power, 
or by large quantities of inflammable air, 
which, on being rarefied by internal fires, 
forces its way through the parts that sur- 
round it. 

EAST, one of the four cardinal points of 
the world ; being that point of the horizon, 
where the sun is seen to rise when in the 
equinoctial. 

ECCENTRICK, in geometry, a term ap- 
plied to circles and spheres which have not 
tho same centre, and consequently are not 
parallel, in opposition to concentrick, where 
they are parallel, having one common cen- 
tre. 

ECCENTRICITY, in astronomy, is the 
distance of the centre of the orbit of a pla- 
net from the centre of the sun, that is, the 
distance between the centre of the ellipsis 
and the focus. 

ECHO, a sound reverberated or reflected 
to the ear from some solid body. 

ECLIPSE, the deprivation of the light of 
the sun, or of some heavenly body, by the 
interposition of another neavenly body be- 
tween it and our sight. 

ECLIPTICK,inriHtronomy,a great circle 
of the sphere, supposed to be drawn through 
the middle of t lie zodiack ; or it is that path 



among the fixed stars, that the earth ap- 
pears to describe, to an eye placed in the 
sun. 

ELASTICITY, that disposition in bo- 
dies by which they endeavour to restore 
themselves to the posture fiom whence 
they were displaced by an external force. 

ELECTRICITY is an invisible, subtile 
fluid, that appears to pervade all nature, 
and among other interesting phenomena, is 
the cause of thunder and lightning. Elec- 
tricity is of two kinds — positive and nega- 
tive. The positive is that state of a body 
which contains more than its due propor- 
tion ; and the negative is that state of a 
bodv which contains less than its due pro- 
portion. When two bodies, one charged 
with positive electricity and the other with 
negative, come in contact with each other, 
so much passes from the former to the lat- 
ter, as to produce an equilibrium — it passes 
thus with a flash and an explosion. Thus 
two clouds, charged in the above manner, 
coming together, or one cloud coming in 
contact with the earth, thunder and light- 
ning are produced. 

ELLIPSIS, in geometry, a curve line re- 
turning into itself,.and produced from the 
section of a cone by a plane cutting both its 
sides, but not parallel to the base. 

EMERSION, in astronomy, is when any 
planet that is eclipsed begins to emerge or 
get out of the shadow of the eclipsing body. 
It is also used when a star, before hidden 
by the sun, as being too near him, begins to 
re-appear or emerge out of his rays. 

EQUATOR is an imaginary circle equal- 
ly distant from the poles, and dividing the 
earth into two equal parts, one being called 
the Northern hemisphere, and the other the 
Southern hemisphere. 

EQUINOCTIAL, in astronomy, a great 
circle of the celestial globe, whose poles are 
the poles of the world. It is so called, be- 
cause, whenever the sun comes to this cir- 
cle, the days and nights are equal all over 
the globe ; being the same with that which 
the sun seems to describe at the time of the 
two equinoxes of spring and autumn. 

EQUINOX, the time when the sun en- 
ters either of the equinoctial points, where 
the ecliptick intersects the equinoctial. 

EXHALATION, a general term for all 
the effluvia or streams raised from the sur- 
face of the earth in form of vapour. Some 
distinguish exhalations from vapours, ex- 
pressing by the former all steams emitted 
from solid bodies, and by the latter, the 
steams raised from water and other fluids. 

EXPANSION, the enlargement or in- 
crease of bulk in bodies, chiefly by means 
of heat. 

EXPLOSION, a sudden and violent ex- 
pansion of an aerial or other elastick fluid, 
by which it instantly throws off any obsta- 
cle that happens to be in the way, some- 
times with incredible force, and in such a 
manner as to produce the most astonishing 
effects. 

EXTENSION, in philosophy, one of the 
common and essential properties of body 



PHILOSOPHICAL TERMS. 



245 



or that by which it possesses or takes up 
aome part of universal space, which is call- 
ed the place of a body. 

FIGURE, in physicks, expresses the sur- 
face, or terminating extremities of any bo- 
dy ; and, considered as a property of body 
affecting our senses, is defined a quality 
which may be perceived by two of the 
outward senses. Thus a table is known 
to be square by the sight and by the 
touch. 

FLUID, in physiology, an appellation 
given to all bodies whose particles easily 
yield to the least partial pressure or force 
impressed. 

FOCUS, in geometry and conick sections, 
is applied to certain points in the parabola, 
ellipsis, and hyperbola, where the rays re- 
flected from ail parts of these curves con- 
cur and meet. 

FOGS are clouds which float on the sur- 
face of the earth, and clouds are fogs in the 
higher regions of the atmospheie ; from 
many places they may be seen floating in 
the vallies, and often in the vallies they 
may be seen creeping along the sides of the 
mountains. 

FORCE, in mechanicks, denotes the cause 
of the change in the state of a body, when, 
being at rest, it begins to move, or has a 
motion which is either not uniform or not 
direct. Mechanical forces may be reduced 
to two sorts, one of a body at rest, the other 
<of a body in motion. 

FORCING-PUMP, in mechanicks, a kind 
of pump ia which there is a forcer or piston 
without a valve. 

FOUNTAIN, in philosophy, a spring or 
source of water rising out of the earth. 

FBICTION, in mechanicks, the rubbing 
of the parts of engines and machines against, 
each other, by which means a great part 
of their effect is destroyed. 

FRLGID ZONES, the spaces on the 
earth's surface between the polar circles 
and the poles. 

FULCRUM, in mechanicks, the press or 
support, by which a lever is sustained. 

GALAXY, in astronomy, a very 
markable appearance, sometimes double, 
but for the most part single, surrounding 
the whole concave of the heavens, called 
the galaxy or milky way. 

GIBBOUS, iu astronomy, a term used in 
.reference to the enlightened parts of the 
moon, whilst she is moving from her first 
quarter to the full, and from the full to the 
last quarter. 

GLOBE, a round or spherical body, more 
usually called a sphere, bounded by one 
uniform convex surface, every point of 
which is equally distant from a point with- 
in called the centre. 

GRAVITY, a term used by physical 
writers to denote the cause by which all 
bodies move towards each other, unless 
prevented by some other force or obstacle. 

GREEN, one of the original colours ex- 
cited by the rays of light. 

HAIL, a compact mass of frozen water, 
eonsisting of such vapours as are united 
Q1 * 



into drops, and are frozen while they are 
falling. They assume various figures, be- 
ing sometimes round, at other times pyra- 
midal, cuniated, angular, thin and flat, and 
sometimes stellated with six radii like the 
imall crystals of snow. 

HALO, in physiology, a meteor in the 
form of a luminous ring or circle, of vari- 
ors colours, appearing round the bodies of 
the sun, moon, or stars. 

HARDNESS, in physiology, is the resist- 
ance opposed by a body to the separation 
of its particles. This property depends on 
the force of cohesion ; and a body is con- 
sidered more hard in proportion as it pre- 
sents a greater resistance to the force which 
may be applied in order to separate its 
parts. 

HARMONY, in musick, the agreeablo 
result, or union, of several musical sounds, 
heaid at one and the same time, or the mix- 
ture of divers sounds, which together have 
an effect agreeable to the ear. As a con- 
tinued succession of musical sounds pro- 
duces melody, so does a continued combi- 
nation of these produce harmony. 

HARMONY OF THE SPHERES, a sort 
of musick much talked of by many of the 
ancient philosophers, supposed to be pro- 
duced by the sweetly tuned motions of the 
stars and planets. This harmony they at- 
tributed to the various proportionate im- 
pressions of the heavenly globes upon one 
another, acting at proper intervals. 

HEIGHT, in geometry, is a perpendicu- 
lar let fall from the vertex, or top, of any 
right-lined figure, upon the base or side 
subtending it. It is likewise the perpendi- 
cular height of any object above the hori- 
zon. 

HEMISPHERE, the half of a globe or 
sphere, when it is supposed to be cut 
hrough its centre in the plane of one of its 
great cireles. 

HORIZON, in astronomy and geography, 
that great circle which divides the heavens 
and the earth into two equal parts or he- 
mispheres, distinguishing the upper from 
the lower. The horizon is either sensible 
or rational — the sensible horizon is that cir- 
cle, which being discovered by our senses, 
limits our prospect. 

HORIZONTAL, something relating to 
the horizon ; or that is taken in, or on a 
level with the horizon. Thus, we say, a 
horizontal plane. 

HURRICANES are violent storms, fre- 
quent in South America and the West In- 
dies, and other hot countries, in which the 
wind changes in a short time to every point 
of the compass, and blows with a violence 
which scarcefy any thing can resist. 

HYADES, in astronomy, seven stars in 
the bull's head, famous among tne poets 
for the bringing of rain. 

HYDRA, in astronomy, a southern con- 
stellation, imagined to represent a water 
serpent. 

HYDRAULICKS teach us to ascertain 
the velocity and impetus of fluids when in 
motion, and serves as the basis fiw eowpui*- 



MG 



A DICTIONARY OF 



ins the power* of rarioua machinery actedjthe place, whose latitude is spoken of, is on 



upon hv running water. 

II YDROM ETEBj an instrument to mea- 
sure the extent and specifick gravity of 
fluids. 

HYDROSTATIC AL BALANCE, a kind 
l f nalance contrived for the easy and exact 
Biting the specifick gravities of bodies both 
liquid and solid. 

IIYDROSTATICAL PARADOX is this 
— t hat any quantity of fluid, however small, 
may be made to balance, or counterpoise 
anv quantity, however large. 

i 1 YDROST AT1CKS treat of the nature, 
gravity, pressure, and motion of fluids in 
general, and of the methods of weighing 
solids in them. 

IMAGE, inopticks, is the appearance of 
an object made either by reflection or re- 
fraction. In all plane mirrors, the image 
is of the same magnitude as the object, and 
it appears as far behind, the mirror as the 
object is before it. In concave mirrors the 
object appears larger, and in those which 
are convex, it appears less than the object. 

IMMERSION, in astronomy, is when a 
star or planet is so near the sun, with re- 
gard to our observations, that we cannot 
see it ; being as it were enveloped or hidden 



this or that side of the equator. 

LATITUDE, in astronomy, the distance 
of a star or planet from the ecliptick, in 
degrees, minutes, and seconds, measured on 
a circle of latitude drawn through that star 
or planet, being either north or south, as 
the object is situated either on the nortli or 
south side of the ecliptick. 

LEE, an epithet to distinguish that half 
of the horizon to which the wind is direct- 
ed from the other part where it arises, 
which latter is accordingly called to wind- 
ward. 

LENS properly signifies a small round- 
ish glass, of the figure of a lentil, but is ex- 
tended to any optick glass, not very thick, 
which either collects the rays of light into 
a point, in their passage through it, or dis- 
perses them further apart, according to the 
laws of refraction. 

LEO, in astronomy, one of the twelve 
signs of the zodiack, the fifth in order. 

LEVEL, an instrument constructed for 
the purpose of ascertaining the exact level 
of any fluid, building, or any other object. 
Levels are of two kinds — the horizontal 
and the perpendicular. 

LEVER, in mechanicks, an inflexible 



in the rays of that luminary. It also de- right line, rod, or Iwam, supported in a sin- 



notes the beginning of an eclipse of the sun 
or moon, when either of those bodies begins 
to be darkened by the shadow of the other. 

IMPENETRABILITY, in philosophy, 
that property of a body whereby it cannot 
be pierced by another ; thus, a body, which 
so fills a space as to exclude all others, is 
said to be impenetrable. 

INCIDENCE, in mechanicks, denotes 
the direction in which one body strikes on 
another. 

INCLINATION, is a word frequently 



gle point on a fulcrum or prop, and used 
for the raising of weights ; being either 
void of weight itself, or at least having 
such a weight as may be commodiously 
counterbalanced. 

LIBRA, the balance, in astronomy, one 
of the twelve signs of the zodiack, the sixth 
in order ; so called, because when the sun 
enters it, the days and nights are equal, as 
if weighed in a balance. 

LIBRATION, in astronomy, an appa- 
rent inequality of the moon's motion, 



used by mathematicians, and signifies the whereby she seems to librate about her 



mutual approach, tendency, or leaning of 
two lines, or planes, towards each other, so 
as to make an angle. 

INCLINED-PLANE, in mechanicks, is 
merely a line or plane that makes an angle 
with the horizon. It is frequently used to 
move weights from one level to another. 

INERTIA, or inactivity, is that proper- 
ty of matter by which it would always con- 
tinue in the same state of rest, or of mo- 



axis, sometimes from the east to the west, 
and now and then from the west to the 
east ; so that the parts in the western limb 
or margin of the moon sometimes recede 
from the centre of the disk, and sometimes 
move towards it, by which means they be- 
come alternately visible and invisible to 
the inhabitmts of the earth. 

LIGHT is that principle, or thing, by 
which objects are made perceptible to our 



tion, in which it was put, unless changedisense of seeing ; or the sensation occasion 
by some external force. led in the mind by the view of luminous ob- 

INTEGRAL, or integrant, appellations jjects. 
given to parts of bodies which are of a si-| LIGHTNING, an electrical explosion, 
milar nature with the whole. Thus, filings) LINE, in geometry, a quantity extended 
of iron have the same nature and properties in length only, without any breadth or 
as bars of iron. thickness, 

INTENSITY, in physlcks, is the degree] L1UU1D, a fluid not sensibly elastick. 
or rate of power or enorgy of any quality, 'the parts of which move on each other, and 
as of heat and cold. lyicld to the smallest impression. 

JUPITER, in astronomy, one of the pri-| LONGITUDE, in geography, is an arch 
mary planets remarkable for its great :of the equator, intercepted between the 
brightness. jfirBt meridian passing through the propos- 

LATITUDE, the distance of a place ed place ; which is always equal to the an- 
from the equator, or an arc of the meridi- gle at the pole, formed by the first meridian 
an intercepted between the zenith of the and the meridian of the place, 
place and the equator. Hence latitude is; LOOKING-GLASSES are nothing but 
•jtfetf northern <>r lotithern, according as plain mirrors of glass, which, being- imper- 



PHILOSOPHICAL TEP.MS. 



247 



vious to the light, reflect the images of 
things placed before them. 

LUNAR, something belonging to the 
moon ; thus we say, lunar month, lunar 
year, lunar dial, or lunar eclipse. 

LUNATION, the time or period from 
one new moon to another — it is called the 
svnodical month. 

" MAGICK LANTERN is an instrument 
used for magnifyin^paintings on glass, and 
throwing their images upon a white screen 
in a darkened room. 

MAGNETISM explains the properties 
of the loadstone, or natural magnet, which 
is a dark coloured and hard mineral body, 
and is found to be an ore of iron, being ge- 
nerally found in iron mines. 

MAGNITUDE, whatever is made up of 
parts locally extended, or that has s everal 
dimensions ; as a line, a surface, or a solid. 

MANOMETER, an instrument to show 
or measure the alterations in the rarity or 
density of the air. 

MARS, in astronomy, the planet that re- 
volves next beyond the earth in our system, 
is of a red fiery colour, and always gives a 
much duller light than Venus, though some- 
times he equals her in size. 

MATHEMATICKS originally signified 
any discipline or learning ; but at present, 
denotes that science which teaches, or con- 
templates whatever is capable of being 
numbered or measured, in so far as it is 
computable or measurable ; and according 
ly is subdivided into arithmetick, which 
has numbers for its object, and geometry, 
which treats of magnitudes. 

MATTER is the general name of every 
substance, that has length, breadth, and 
thickness. 

MECHANICKS, is the science which 
treats of the laws of the equilibrium and 
motion of solid bodies ; of the forces by 
which bodies, whether animate or inani- 
mate, may be made to act upon one ano- 
ther ; and of the means by which these may 
be increased, so as to overcome such as are 
most powerful. 

MEDIUM, in philosophy, that space or 
region through which a body in motion 
passes to any point ; thu3 ether is suppos- 
ed to be the medium through which tlie 
heavenly bodies move ; air, the medium 
wherein bodies move near the earth ; wa- 
ter, the medium wherein fishes live and 
move; and glass is also a medium of light, 
as it affords it a free passage. 

MELODY, in musick, the agreeable ef- 
fect of different sounds, ranged and dispos- 
ed in succession,; so that melody is the ef- 
fect of a single voice or instrument, by 
which it is distinguished from harmony. 

MERCURY, in astronomy, is a small 
star that emits a very bright white light— - 
though, by reason of his always keeping 
near the sun, he is seldom to be seen ; and! 
when he does make his appearance, his 
motion towards the sun is so swift, that he 
can only be discerned for a short time. 

MERIDIAN, in astronomy, a great cir- 
cle passing through the polos of the world. 



and both zenith and nadir, crosses the equi- 
noctial at right angles, and divides the 
sphere into two hemispheres, the eastern 
and the western ; it has its poles in the 
east and west points of the horizon. It is 
called meridian, because, when the sun 
comes to the south part of this circle, it 
is then mid-day ; and then the sun has his 
greatest altitude for that day. 

METEOR, in physiology, a moveable ig- 
neous body, congregated in the air by means 
not thoroughly understood, and varying 
greatly in size and raoiditv of motion. 

METEOROLOGY* is the science of 
studying the phenomena of the atmo- 
sphere, and that term by which is expressed 
all the observations that tend to make them 
a system. 

MICROSCOPE, in opticks. By micro- 
scopes are understood instruments, of what- 
ever structure or contrivances, that can 
make small objects appear larger than they 
do to the naked eye. 

MINUTE, in geometry, the sixtieth part 
of a degree of a circle. Minutes are denot- 
ed by one acute accent, thus (') ; as the se- 
cond, or sixtieth part of a minute, is by 
two such accents, thus (") ; and the third 
by three ("'). 

MIRRORS, in catopticks, any polished 
body impervious to the rays of light, and 
which reflects them equally. Mirrors were 
anciently made of metal ; but at present 
they are generally smooth plates of glass, 
tinned or quick-silvered on the back part, 
and called looking-glasses. The doctrine 
of mirrors depends wholly on that funda- 
mental law, that the angle of reflection is 
always equal to the angle of incidence. 

MOBILITY is that property of matter 
by which it is capable of being moved from 
one part of space to another. 

MOMENTUM, in mechamcks, signifies 
the same with impetus, or quantity of mo- 
tion in a moving body ; which is always 
equal to the quantity of matter multiplied 
into the velocity ; or, which is the same 
thing, it may be considered as a rectangle 
under the quantity of matter and velocity. 

MONSOON, in physiology, a species of 
wind , in the East Indies, which for six 
months blows constantly the same way, 
and the contrary way the other six months. 

MOON, in astronomy, a satellite, or se- 
condary planet, always attendant on our 
earth. 

MOTION is defined to be the continued 
and successive change of place. Nothing 
can be produced or destroyed without mo- 
tion, and every thing that happens depends 
on it. 

MUSICK. Any succession of sounds, 
however much they may vary in regard to 
duration, or however much they may par- 
take of various modes or keys, provided that 
succession be agreeable, and excites, in a 
well tuned ear, certain agreeable sensa- 
tions, is called musick. 

NADIR, in astronomy, that point of the 
heavens which is diametrically opposite to 
the zenith, 01 point directly over our heads. 



•248 



A DICTIONARY OP 



The zenith and nadir are the two poles of 
the horizon. 

NATURAL PHILOSOPHY, otherwise 
called physicks, is that science which con- 
siders the powers of nature, the properties 
of natural bodies, and their actions upon 
one another. 

NEBULAE, in astronomy, luminous spots 
in the heavens, some of which consist of 
clusters of telescopick stars, others appear 
asY.uminous spots of different forms. Some 
of them form a round compact system, 
others are more irregular, of various forms, 
and some are long and narrow. 

NIGHT, that part of the natural day 
during which the sun is underneath the 
horizon ; or that space wherein it is dusky. 

NODES, in astronomy, the two points 
Wherein the orbit of a planet intersects the 
ecliptick, whereof the node, where the node 
ascends northwards, above the plane of the 
ecliptick, is called the ascending node; and 
the other, where the planet descends to the 
south, is called the descending node. 

OBLATE, flattened, or shortened, as an 
oblate spheroid, having its axis shorter 
than its middle diameter, being formed by 
the rotation of an ellipse about the shorter 
axis. The oblateness of the earth refers to 
the diminution of the polar axis in respect 
of the equatorial. 

OBTUSE, signifies blunt or dull, in op- 
position to sharp or acute. Thus we say 
an angle is obtuse if it measures more than 
ninety degrees. 

OCCIDENT, in geography, the western 
quarter of the horizon, or that part of the 
horizon where the ecliptick, or the sun 
therein, descends into the lower hemi- 
sphere, in contradistinction to orient. 

OCCULTATION, in astronomy, the 
time a star or planet is hidden from our 
sight, by the interposition of the moon or 
of some other planet. 

OPACITY, in philosophy, a quality of 
bodies which renders fuem impervious to 
the rays of li?ht. 

OPTICKS, the science of vision, includ- 
ing Catoptricks and Dioptricks, and even 
Perspective ; as also the whole doctrine of 
light and colours, and all the phenomena 
of visible objects. 

ORBIT, in astronomy, the path of a pla- 
net or comet, or the curve that it describes 
in its revolution round its central body. 
Thus the earth's orbit is the curve which 
it describes in its annual course, and usu- 
ally called the ecliptick. 

ORION, in astronomy, a constellatron of 
the southern hemisphere, consisting of 
thirty-seven stars, according to Ptolemy: 
of sixty-two, according to Sycho ; and of 
no less than eighty, in the Britannick cata- 
logue. 

ORRERY, a curious machine for repre- 
senting the motions and appearances ofthe 
heavenly bodies. 

OSCILLATION, in mechanicks, the vi- 
bration or reciprocal ascent and descent of 
a pendulum. 

PARABOLA, in geometry, a figure aris- 



ing from the section of a cone, when cut by 
a plane parallel to one of its sides. 
! PARADOX, in phi losophy , a proposition 
'seemingly obscure, as being contrary to 
some received opinion, but yet true in fact. 

PARALLAX, in astronomy, denotes a 
change of the apparent place of any hea- 
venly body, caused by being seen from dif- 
ferent points of view; or it is the difference 
between the true and apparent distance of 
any heavenly body fromrthe zenith. 

PARALLEL straight lines, whose least 
distances from each other are every where 
equal, are said to be parallel. 

PARALLELOGRAM, in geometry, a 
quadrilateral right lined figure, whose op- 
posite sides are parallel and equal to each 
other. 

PARHELIUM, or PARHELION, in 
phvsiology, a mock sun, or meteor, in form 
of a very bright light, appearing on one 
~ide of the sun. 

PEGASUS, in astronomy, a constellation 
of the northern hemisphere, in form of a 
flying horse. 

PENDULUM, in mechanicks, denotes 
any heavy body so suspended as that it 
may vibrate or swing backwards and for- 
wards, about some fixed point, by the force 
of gravity. The vibrations of the pendu- 
lum are called its oscillations. 

PENUMBRA, in astronomy, a partial 
shade observed between the perfect shadow 
and the full light, in an eclipse. 

PERCUSSION, in mechanicks, the im- 
pression a body makes in falling or striking 
upon another, or the shock of two bodies 
in motion. 

PERIHELIUM, in astronomy, that point 
of a planet's or comet's orbit wherein it is 
in its least distance from the sun ; in which 
sense it stands in opposition to aphelium. 

PERIMETER, in geometry, the bounds 
or limits of any figure or body. The peri- 
meter of surfaces or figures are lines, those 
of bodies are surfaces. In circular figures, 
instead of perimeter, we say circumference, 
or periphery. 

PERIOD, in astronomy, the time taken 
up by a star or planet in making a revolu- 
tion rouud the sun ; or the duration of its 
course till it return to the same point of its 
orbit. 

PERIPHERY, in geometry, the circum- 
ference of a circle, ellips.J, or any other re- 
gular curvilinear figure. 

PERPENDICULAR, in geometry, a line, 
falling directly on another line, so as to 
make equal angles on each side ; called 
also a normal line. 

PERSPECTIVE, the art of represent- 
ing, upon a plane surface, the appearance 
of objects, however diversified, similar to 
that they assume upon a glass-pane, inter- 
posed between them and the eye at a given 
distance. 

PHASES, in astronomy, the several ap 
pearances or quantities of illumination of 
the Moon, Venus, Mercury, and the othei 
planets ; or the several manners whereii 
they appear illuminated by the sun. 



PHILOSOPHICAL TERMS. 



249 



PHOENIX, in astronomy, one of the 
constellations of the southern hemisphere, 
unknown to the ancients, and invisible in 
our northern parts. It is said to consist of 
thirteen stars. 

PHYSICKS, a term made use of for na- 
tural philosophy, explains the doctrines of 
natural bodies, their phenomena, causes, 
and effects, with the various effections, 
motions, and operations. 

PISTON, in pump-work, is a short cylin- 
der of metal, or other solid substance, fitted 
exactly to the cavity of the barrel or body 
of the pump. There are two kinds of pistons 
used in pumps, the one with a valve, and 
the other without a valve, called a forcer. 

PLANE, in geometry, denotes a plain 
surface, or one that lies evenly between its 
bounding lines — and as a right line is the 
shortest extension from one point to ano- 
ther, so a plain surface is the shortest exten- 
sion from one line to another. 

PLANET, a celestial body revolving 
round the sun, as a centre, and continually 
changing its position, with respect to the 
fixed stars ; whence the name planet, which 
is a Greek word signifying wander. 

PLEIADES, in astronomy, an assem 
blage of seven stars in the neck of the con 
stellation Taurus, the bull ; although there 
are now only six of them visible to the na- 
ked eye. The largest is of the third ma; 
nitude, called " Lucido pleiadum." 

PNEUMATICKS is that branch of 
natural philosophy which treats of the 
weight, pressure, and elasticity of the air, 
with the effects arising from them. 

POINT, in geometry, as defined by Eu- 
clid, is a quantity, which has no parts, or 
which is indivisible. Points are the ends 
or extremities of lines. If a point be sup- 
posed to be moved any way, it will, by its 
motion, describe a line. — Point, in physicks, 
is the least sensible object of sight, marked 
with a pen, point of a compass, or the like. 
Of such points all physical magnitude 
consists. 

POLAR, in general, something relating 
to the poles of the world, or poles of arti 
ficial globes. 

POLARITY, the quality of a thing con- 
sidered as having poles ; but chiefly used 
in speaking of the magnet. 

POLE, in astronomy, one of the extre- 
mities of the axis, on which the sphere re- 
volves. These two points, each ninety de- 
grees from the equinoctial or equator, are 
by way of eminence called the poles of the 
world ; and the extremities of the axis of 
artificial globes, corresponding to these 
points in the heavens, are termed the poles 
thereof. 

POLLUX, in astronomy, a fixed star of 
the second magnitude in the constellation 
gemini, or the twins. The same name is 
also given to the hindermost twin, or pos 
terior part of the same constellation. 

POWER, in mechanicks, denotes any 
force, whether of a man, a horse, a spring, 
the wind, or water, which being applied to 
a machine, tends to produce motion. 



PRECESSION OF THE EQUINOXES 

is a very slow motion of them, by which 
they change their place, going from east to 
west or contrary to the order of the signs. 
PROJECTION, in mechanicks, the art 
of communicating motion to a body, from 
thence called projectile. 

PULLEY, in mechanicks, one of the 
mechanical powers, called by seamen a 
tackle. 

PUMP, in hydraulicks, a machine formed 
on the model of a syringe, for raising water. 
PYROMETER, an instrument for mea- 
suring the expansion of bodies by heat. 

QUADRANT denotes a mathematical 
instrument, of great service in astronomy, 
and consequently, in navigation, for taking 
the altitudes of the sun and stars, as also 
for taking angles in surveying. 

QUADRATURE, in geometry, denotes 
the squaring or reducing a figure to a 
square. 

QUADRILATERAL, in geometry, a 
figure whose perimeter consists of four 
right lines making four angles ; whence it 
is also called a quadrilateral figure. The 
quadrilateral figures are either a parallelo- 
gram, trapezium, rectangle, square, rhom- 
bus, or rhomboides. 

RADIATION, the act of a body emitting 
or diffusing rays of light all around, as 
from a centre. 

RADIUS, in geometry, the semi-diame- 
ter of a circle, or a right line drawn from 
the centre to the circumference. 

RAIN. Whatever suddenly disturbs the 
heat or density of the air, or the electricity 
of the clouds, occasions the particles of 
vapour to rush together, and form drops of 
water too heavy to continue suspended in 
the atmosphere. They then fall in the 
shape of rain, and increase in size as they 
fall by combining with the floating vapours 
as they pass through them. 

RAINBOW is a meteor in form of a 
party-coloured arch, or semicircle, exhibit- 
ed only at the time when it rains. It is 
always seen in that point of the heavens 
which is opposite to the sun, and is occa- 
sioned by the refraction and reflection of 
his rays in the drops of falling rain. 

RAREFACTION, in physicks, is the 
making a body to expand, or occupy more 
room or space, without the accession of 
new matter. 

RAY, in opticks, a beam of light, emitted 
from a radiant or luminous body. 

REACTION, in physiology, the resist- 
ance made by all bodies to the action or 
impulse of others, that endeavour to change 
its state, whether of motion or rest. 

RECEIVER, in pneumaticks, a glass 
vessel for containing the thing on which 
an experiment in the air pump is to be 
made. 

RECTANGLE, in geometry, the same 
with a right angled parallelogram. 

REFRACTION, is the deviation of a 
moving body from its direct course, occa- 
sioned by the different density of the medi- 
um in which it moves ; or, it is a change 



250 



A DICTIONARY OP 



of direction, occasioned by a body's falling 
obliquely out of one medium into another 
of a different density. 

REPULSTON, in physicks, that property 
in bodies, whereby, if they are placed just 
beyond the sphere of each other's attraction 
of cohesion, they mutually fly from each 
other. 

RESISTANCE, in philosophy, denotes, 
in general, any power which acts in an op- 
posite direction to another, so as to destroy 
or diminish its effects. 

RETINA, the expansion of the oplick 
nerve on the internal surface of the eye, 
whereupon the images of objects being 
painted, are impressed, and by that means 
conveyed to the common sensory in the 
brain, where the mind views and contem 
plates their ideas. 

ROTATION, iu geometry, a term chiefly 
applied to the circumvolution of any sur- 
face round a fixed and immoveable line, 
which is called the axis of its rotation, and 
by such rotations it is that solids are con 
ceived to be generated. 

SAGITTARIUS, the archer, in astrono- 
my, the ninth sign of the zodiack. 

SATELLITES, in astronomy, are cer 
tain secondary planets, moving round the 
other planets, as the Moon does round the 
Earth. They are so called, because they 
always attend them, and make the tour 
about the sun with them. 

SATURN is a very conspicuous planet, 
though not so brilliant as Jupiter. 

SEGMENT OF A CIRCLE, in geometry,, 
that part of the circle contained between a 
chord and an arch of the same circle. 

SEMICIRCLE, in geometr), half a cir 
cle, or that figure comprehended between 
the diameter of a circle and half the cir 
eumference. 

SEMIDIAMETER, half the diameter, 
or a right line drawn from the centre of a 
circle, or sphere, to its circumference ; be- 
ing the same with what is otherwise called 
the radius. 

SEXTANT, in mathematicks, denotes 
the sixth part of a circle, or an arch com 
prehendi ng sixty degrees. The word sex 
tant is more particularly used for an astro- 
nomical instrument made like a quadrant, 
excepting tbat its Jimb only comprehends 
sixty degrees. The use and application of 
the sextant is the same with that of the 
quadrant. 

SHADOW, in opticks, a privation or di 
minution of light by the interposition of an 
opaque body ; or it is a plane, where the 
light if either altogether obstructed, or 
greatly weakened, by the interposition of 
some opaque body between it and the lumi 
nary. 

SIDEREAL DAY, is the time in which 
any star appears to revolve from the meri 
dian to the meridian again. 

SIGNS, in astronomy. The ecliptick is 
usually divided, by astronomers, into 12 
parts called signs, each of which of course 
contains 30 degrees. They are usually 
called Bigns of the zodiack ; and beginning 



at the equinox, where the Sun interiecti 
and rises above the equator, have these 
names and marks : 

Aries, <p Leo, g\ Sagittarius, / 

Taurus, y Virgo, JTJ) Capricornus,)/f 

Gemini, rj Libra, ^ Aquarius, <» 

Cancer, cr^ Scorpio, ]\[ Pisces, ^ 

Of these signs, the first six are called north 
em, lying on the north side of the equator 
and the last six are called southern, being 
situated to the south of the equator. 

SIPHON, or Syphon, in hydraulicks, a 
bended pipe, one end of which being put 
into a vessel of liquor, and the other hang 
ing out of the said vessel over another, the 
liquor will run out from the first into the 
last, after the air has been sucked out of 
the external or lower end of the siphon, 
and that as long as the liquor in the upper 
vessel is above the upper orifice of the si- 
phon. 

SKY, the blue expanse of air and atmo- 
sphere. The azure colour of the sky is at- 
tributed, by Sir Isaac Newton, to vapours 
beginning to condense there, and^which 
have got consistence enough to reflect the 
most flexible rays. 

SNOW, a well known substance, formed 
by the freezing of the vapours in the at- 
mosphere. It differs from hail and hoar- 
frost, in being as it were crystallized, which 
they are not. 

SOLID, in philosophy, a body whose 
parts are so firmly connected together, as 
not to give way or slip from each other up- 
on the smallest impression ; in which sense 
solid stands opposed to fluids. 

SOLAR, something belonging to the sun ; 
thus the solar system is that system of the 
world wherein the heavenly bodies are 
made to revolve round the sun as the cen- 
tre of their motion. 

SOLSTICE, in astronomy, that time 
when the sun is in one of the solstitial 
points ; that is, when he is at his greatest 
distance from the equator, thus called, be- 
cause he then appears to stand still, and 
not to change his distance from the equa- 
tor for some time; an appearance owing 
to the obliquity of our sphere, and to which 
those living under the equator are stran- 
gers. 

SOUND. The sense of hearing is affect 
ed by the pulsations or vibrations of the 
air, which are caused by its own expan- 
sion, or by the vibrations of sounding bo- 
dies. These sensations, or vibrations in 
the air, are called sounds, as are also the 
sensations which thev produce. 

SPECIF1CK, in philosophy, that which 
is peculiar to any thing, and distinguishes 
it from all others. 

SPECTRUM, in opticks. When a ray 
of light is admitted through a small hole, 
and received on a white surface, it forms a 
luminous spot. If a dense, transparent bo- 
dy be interposed, the light will be refracted, 
in proportion to the density of the medium ; 
but it a triangular glass prism be inter 



PHILOSOPHICAL TERMS. 



351 



posed, the light is not merely refracted, 
but it is divided into seven different rays. 
This image is called the spectrum, and 
from its being produced by the prism, the 
pnsmatick spectrum. 

SPHERE is a solid contained under one 
uniform round surface, such as would be 
formed by the revolution of a circle about 
the diameter thereof, as an axis. 

SPHEROID, in geometry, a solid, ap- 
oroaching to the figure of a sphere. 

SPOTS, in astronomy, certain places of 
the Sun's or Moon's disk, observed to be 



THUNDER, the noise occasioned by the 
explosion of a flash of lightning passim? 
through the air ; or it is that noise which 
is excited by a sudden explosion of electri 
cal clouds which are therefore called thun- 
der clouds. 

TORRID ZONE, among geographers, 
denotes that space of the earth's surface 
included between the tropicks. 

TRADE WINDS denote certain regular 
winds at sea, blowing either constantly 
the same way, or else alternately, a certain 
length of time in one direction, and then 



either more bright or darker than the rest,'as long in an opposite one. They are call- 
and accordingly called facula and macula. |ed trade winds from their use in navigation, 

SPRAY, the sprinkling or foam of the 'and are very common in the Indian seas, 
sea, which is driven from the top of a TRANSIT, in astronomy, signifies the 
wave in stormy weather. 'passage of any planet just by, or over, a 

SQ,UAE.E, in geometry, a quadrilateral fixed star, or sun, and of the moon in par- 
figure, both equilateral and equiangular, jticular, covering or moving over any planet. 

STAR, in astronomy, a general name forj TRANSMISSION, in opticks, the act of 
all the heavenly bodies which are dispersed a transparent body passing the rays of 
throughout the whole heavens. i light through its substance, or suffering 

SUCTION, the act of sucking or draw-' them to pass; in which sense the word 
ing up a fluid, as air, water, milk, or thejstands opposed to reflection, 
like, by means of the mouth and lungs. j TRANSPARENCY, in physicks, a qua- 

SUN, in astronomy, the most conspicu- lity In certain bodies, whereby they give 
ous of the heavenly bodies, which occupies passage to the rays of light, in contradis- 
the centre of the system which compre-.tinction to opacity, or that quality of bo- 
hends the earth, the primary and secondary dies which renders them impervious to the 



planets, and the comets. 

SUPERFICIES, or surface, in geometry, 
a magnitude considered as having two di- 
mensions ; or extended in length and 
breadth, but without thickness or depth. 



rays of light. 

TRIANGLE, in geometry, a figure of 
three sides and three angles. 

TROPICKS, in astronomy, and geogra 
phy, are two circles supposed to be draws 



SWIMMING, the art or act of sustain-j round the earth on each side of the equa- 
ing and moving the body in water. Brutes' tor, and 23 deg. 29' distant from it. 
swim naturally, but men attain this art by! TWILIGHT, that light, whether in the 
practice and industry. It consists princi- morning before sunrise, or in the evening 
pally in striking the water alternately with'after sunset, which is occasioned by the 
the hands and feet, which, like oars, row a reflection of the sun's rays in passing 



person forward. 

SYRINGE, an instrument serving to im- 
bibe or suck in a quantity of any fluid, and 
to squirt or expel the same with violence. 

S\ ZYGY, in astronomy, a term equally 



through the atmosphere. 

VACUUM, in philosopny, denotes \ 
space empty or devoid of all matter or body 

VALVE, in hydraulicks and pneuma 
ticks, is a kind of" lid or cover, of a tube or 



used for the conjunction and opposition of j vessel, so contrived as to open one way ; 
a planet with the sun. {but which the more forcibly it is pressed 

TANGENT, in geometry, is defined, in the other way, the closer it shuts the aper- 
general, to be a right line, which touchesture, so that "it either admits the entrance 
any arch of a curve, in such a manner, as of a fluid into the tube or vessel, and pre- 



to make a right angle with the diameter of 
the circle of which that arch is a part. 

TANTALUS' CUP, in hydraulicks, a 
siphon, so adapted to a cup, that the short 
leg being in the "up, the long leg may go 
down through the bottom of it. 

TAURUS, in astronomy, one of the 
twelve signs of the zodiack, the second in 
order, consisting of forty-four stars, accord- 
to Ptolemy; of forty-one, according to 



vents its return, or admits its escape, and 
prevents its re-entrance. 

VAPOUR, in meteorology, a thin, humid 
matter, which, being rarefied to a certain 
degree by the action of heat, ascends to a 
particular height in the atmosphere, where 
it is suspended, until it returns in the form 
of dew, rain, snow, or hail. 

VELOCITY, swiftness, or that affection 
of motion, whereby a moving body is dis- 



Tycho ; and of no less than one hundred; posed to run over a certain space in a cer- 

and thirty-five, according to the Britannick'tain time. 

catalogue. I VENUS, the most beautiful star in the 

TELESCOPE, an optical instrument, heavens, known by the names of the mom- 
which is used for discovering and viewing ing and evening star, likewise keeps near 
distant objects, either directly by glasses,!the sun, though she recedes from him al- 
or by reflection. [most double the distance of Mercury. 

THERMOMETER, an instrument for! VESTA, one of the small planetary bo- 
measuring the degree of heat or cold in any dies discovered lately to revolve between 
"°dy. the planets Mars and Jupiter. 



252 



A DICTIONARY OF PHILOSOPHICAL TERMS. 



VIBRATION, in mechanicks, a regular 
reciprocal motion of the body, as, for ex- 
ample, a pendulum, which, being freely sus- 
pended, swings or vibrates from side to 
side. 

VIRGO, in astronomy, one of the signs 
or constellations of the zodiack, and the 
sixth according to order. 

VISIBLE, something that is an object 
of sight or vision, or something whereby 
the eye is affected, so as to produce a sen- 
sation. 

VISION is the act of seeing or of per- 
ceiving external objects by the organ of 
sight. 

UNDULATION, in physicks, a kind of 
tremulous motion or vibration observable 
in a liquid, whereby it alternately rises and 
falls like the waves of the sea. 

UNISON, in musick, the effect of two 
sounds which are equal in degree of tune, 
or in point of gravity and acuteness. 

VOLCANOES, mountains which emit 
ignited matter and smoke through aper- 
tures, communicating with cavities in the 
depths of the earth. 

WATER, a transparent fluid, without 
colour, smell, or taste, in a very small de- 
gree compressible ; and, when pure, not 
. iable to spontaneous change. 

WATER SPOUT, an extraordinary me- 
teor, in which a column of water is seen 
hanging from the clouds, and descending 
until it meets with a column rising from 
the ocean. They unite and often move 
with rapidity, until they meet with some 
opposing wind, or other cause, which de- 
stroys them. 

WAVE, in physicks, a volume of water 
elevated by the action of the wind, upon its 
surface, into a state of fluctuation, and ac- 
companied by a cavity. 

WEDGE, one of the mechanical powers, 
as they are called. The wedge is a trian- 



gular prism, whose bases are equilateral 
acute angled triangles. 

WEEK, in chronology, a division of 
time comprising seven days. 

WEIGHT, in physicks, is a quality in 
natural bodies, by which they tend towards 
the centre of the earth. 

WHEEL, one of the six powers of me- 
chanism ; and, without doubt, contributes 
more than any of the other five to the ge- 
neral convenience of mankind, by the won- 
derful variety of purposes, from a mill to a 
watch, wherein it is employed. 

WHIRLWINDS are formed by opposite 
winds meeting and moving swiftly in a cir 
cle, raising sand and light bodies into tho 
air. In the deserts of Africa they some- 
times draw up the sand into a moving pil- 
lar, which buries all in its way. When 
they appear on the ocean, they draw up the 
water, and produce water-spouts. 

WIND. When the air over any place 
is more heated than that around, it is rare- 
fied or expanded, and rises. The surround- 
ing air rushes in to supply its place, and 
this produces a current called wind. 

YEAR, the time that the sun takes to go 
through the twelve signs of the zodiack. 

ZENITH, in astronomy, the vertical 
point ; or a point in the heavens directly 
over our heads. The zenith is called the 
pole of the horizon, because it is ninety de- 
grees distant from every point of that cir- 
cle. 

ZODIACK, in astronomy, a broad circle, 
whose middle is the ecliptick, and its ex- 
tremes, two circles, parallel thereto, at 
such a distance from it, as to bound or 
comprehend the excursions of the sun and 
planets. 

ZONE, in geography and astronomy, a 
division of the terraqueous globe, with re- 
spect to the different degrees of heat found 
in the different parts of it 



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